One of the things that I do a lot of as part of my professional life is write industry standards. I do this as part of several committees in the aviation industry that are responsible for creating documents that, after going through an extensive consensus and vetting process, are more often than not accepted by governmental authorities around the world as acceptable means of complying with various parts of their Regulations. (I've also been involved with rewriting some of those Regulations). Personally, I really enjoy this sort of work, though a lot of my colleagues think there is something a bit off with me as a result. To me, it’s a fascinating puzzle - how to use the English language to write what you actually mean in such a way that someone you’ve never met can read those words 1,5, 10 years from now and know exactly what you had in mind: no more, no less. Since this isn’t the only context in which this sort of communication is important, and since it is very different from a lot of other writing that people have to do, here are ten tips for making your meaning durably clear (and clearly durable):

Use must, should, could, and friends appropriately.
The devil is truly in the details. Words like “may”, “must”, “should”, “shall”, “can”, and “could” might be used relatively interchangeably in day-to-day communications. When writing regulations or standards, these words mean very different things. “Must” and “shall” connote a directive: “all airplanes must be painted blue” or “you shall paint your airplane blue” are different from “all airplanes should be painted blue [but green is okay too]” or “you could paint your airplane blue [or green, or magenta, or orange]”. If it would be acceptable to use something other than “must” or “shall”, consider carefully whether you need the statement at all. What you put in the document may very well end up as part of a liability trial, or at the least will likely influence certification and compliance costs, so unsolicited advice is not welcome.
Use ie., e.g., and etc. appropriately.
Another detail that can have devilish repercussions if not used correctly are these little abbreviations that we all throw around in daily writing. The biggest difference to keep in mind is that “ie.” and “e.g.” mean very different things. If you use “ie.”, that indicates that what follows is the only acceptable alternative interpretation or example. If you use “e.g.”, that indicates that what follows are examples but not an exhaustive list thereof. Tacking “etc.” onto the end of a list means you probably should have just started it with “e.g.” and been done with it. Don’t use “e.g.” and “etc.” in the same list as that is redundant. And don’t use “i.e.” and “etc.” in the same list as that is contradictory.
Use Oxford commas.
Wars have been fought over issues inspiring less passion than the Oxford comma. Whatever you choose to do in your personal life, I’m here to tell you that if you’re trying to write for clarity, you need to use the Oxford comma. For those of you who are new to the Oxford comma (also called a series or serial comma) saga, it is the comma that precedes the “and” in a list: “the test must be done using water, sand, and salt spray” means that you’re doing three test; “the test must be done using water, sand and salt spray” means that you’re doing two - and that the last one is going to be a very different test than any of the three in the first example.
Employ one meaning per word and one word per meaning.
This is another area where your personal preference for writing style has to be deprioritized in favor of clarity. We’re often taught in middle school English classes to vary our phrasing and not just use the same word all the time. When Joe leaves his house, he may walk one time, but he may stroll, saunter, skip, or shuffle the next. In standards and regulatory writing, each word must always mean the same each time it is used AND, more insidiously, each time you mean the same thing you have to use the same word. No points for creativity. So pick the clearest and most appropriate word for a given meaning, and stick to it. Every time Joe leaves the house, unless there is a deliberate and valuable reason to express it differently, he just walks out the door.
Make it achievable and pay attention to implied tolerances.
It’s very tempting to use words like “minimize” or ”maximize”, but they don’t actually mean anything concrete. Minimizing or maximizing something is, technically speaking, a never ending process. It can theoretically always be a little bit more or less, no matter whether that incremental change has a tangible impact on the underlying intent of the requirement. If you’re tempted to use one of these open-ended words, ask yourself what it is that’s keeping you from putting down a clear “good enough” level: do you need research or industry experience to come up with a number? Or can you rephrase your concern in a more defined way? It’s tempting to solve this problem by specifying a possible solution, but it’s better to see item 6 and keep working on alternative ways of phrasing the intent of the requirement. If you are including a number, be aware that adding extra significant figures (as in, more numbers that aren’t zero: 3000 vs. 3145) adds cost for anyone trying to use the document you’re writing. If it doesn’t have to be accurate to that tenth of an inch (or quarter of a pound), don’t specify the number at that level of precision. Be particularly aware of this particular trap when it comes to unit conversions: 5 pounds may be equivalent to 2 kilograms from a standards drafting perspective.
Separate intention from implementation.
This point moves beyond semantics into the philosophy of standards and regulatory writing. It’s often easier to think about what should be in the requirements or standards document based on how you expect it to be applied. There’s nothing wrong with this thought process as part of the exploration and drafting stage, but before the wording is finalized, you need to ask yourself if what you’re requiring is actually the underlying intention, or a requirement for a particular solution that could meet that intention, but may not be the only possible way of achieving the desired objective. For instance, you could specify that the soup “must stirred every two minutes by a wooden spoon”, or you could specify that the soup “must be allowed to heat evenly while being prevented from sticking to the bottom of the container in which it is being heated in a manner that will not permanently mark or otherwise damage said container”. It may seem like the first example is simpler, but it imposes all sorts of unintended requirements that can get in the way of alternative but completely acceptable methods for making lunch. What about a silicone spatula? What about swirling the whole thing? What about one of those nifty magnetic automatic stirring dodads from chemistry lab? The point of a requirement, standard, or regulation is to ensure that a minimum intent is met, not to specify how someone must meet that intent. This is necessary to allow those using the document to both innovate and manage cost. It also increases the longevity of the document by separating it from the current state-of-the-art technology. It helps to ask “why would you do this?” if you’re stuck on an implementation.
Use “approved”, “appropriate”, or “acceptable”, etc. with caution.
Words that imply a thumbs-up from an entity outside the bounds of the document in question come with a lot of baggage and can be very loaded. If you need to use one of these words, do so very explicitly. The following questions might help define the situation more clearly: Who is doing the approval? Could you instead use a phrase like “the latest published revision of document XYZ”? What would make something appropriate/acceptable or not? What is the underlying source of ambiguity that is driving you to want to hand-off the responsibility of setting the requirement and is there any way to rephrase to get at the main point explicitly? These words do make it into standards, regulations, and other requirement documents, and sometimes they are unavoidable, but they should be seen as red flag in the drafting process.
Focus on what is minimally acceptable.
This is closely related to #5, but adds - or rather often removes - a layer. Maybe with our soup example it doesn’t have to heat evenly, but it does have to all be at a warm-but-not-too-hot temperature before it is served. So perhaps it would be better to specify a maximum temperature of any part of the soup when it is served, and leave the cooking technique out of it. Market forces will likely drive people who prepare soup to ensure that their product is uniformly warm, not clumpy, and otherwise rather yummy. As a regulator or standards writer, that’s really not up to you - keeping the soup eater from getting scalded is, however, a minimally acceptable requirement from a safety perspective. This sort of thinking drives the separation of what would be “nice” from what is truly necessary and can make a difference in whether the requirements are complied with or whether they are seen as meddlesome and skirted.
Think about the costs and benefits to all stakeholders..
Lean standards that are well written and focused on safety intent without being overly prescriptive are often welcome by an industry. Clear, clean standards that describe what is minimally acceptable without prescribing how to do it afford manufacturers some level of liability protection without unduly driving costs or hampering innovation. (The “how” can and often is included in guidance or “best practice” documents which can also be extremely useful without implying requirements.) Everything that ends up in a standard or regulatory document can end up having significant long term impacts on how the industry develops: it’s up to those writing them to think through those potential impacts and the cost/benefit trade associated with each one. Part of this thought process should be what the potential costs and/or risks are of imposing requirements that could prevent a new technology from coming to market: if an unattainable requirement is set, will the Consumer be missing out on an important safety or other benefit?
Less is more.
If you can take it out, do. These sorts of documents tend to take on water over the course of their life, as in they will usually organically get more complicated as time goes by but very rarely do they undergo a dramatic simplification. For this reason, and because everything included in a standard or regulation or requirement will end up driving cost and imposing restrictions for someone at some point, if there’s not complete confidence that a particular point should be included, leave it out. Also, be efficient with words. Colloquialisms, jargon, expressions intended to make something more readable, and general niceties are just in the way. The passive voice is also not your friend. Craft simple, direct, unembellished sentences that use the minimum number of words necessary.