Rule authors are faced with the choice between a, each, every, all, and any, just about every time they write a new business rule in English. This is tiresome and it makes it hard for them to be consistent.
A business rule tells you about the desirable business conduct in a particular situation. The rule applies each time that situation occurs. That’s why it is so often possible in business rules to use all, every, each or any. The following 6 versions actually mean the same thing (but the version with any is slightly more emphatic):
A customer who spends over € 50,000 must be assigned a Personal Coach.
Each customer who spends over € 50,000 …
Every customer who spends over € 50,000 …
All customers who spend over € 50,000 …
Any customer who spends over € 50,000 …
Customers who spend over € 50,000 …
So which of these 6 alternatives is best if you are looking for standardisation?
It must be clear that your rule applies to each individual case, independently of other cases. This, I think, puts the versions with plural form at a disadvantage. With Customers…, readers might just think that a group of customers spending € 50,000 together would also apply for a Personal Coach.
Any introduces an element of emphasis that is unnecessary. Each has a slight advantage over every: it conveys the meaning of “each individual case independently” more clearly.
That leaves us with a and each. Personally, I prefer a, though I admit it’s only a matter of style. Each, in a rule, could imply that you can also have rules that do NOT apply to ‘each’ case, which doesn’t make sense given the very nature of business rules. And a is a subtle reminder of the close kinship between business rules and their underlying definitions, since English definitions also typically use a:
A senior passenger is a passenger who is over 64 years old.
Occasionally, you get rules that contain no restrictive condition at all. They look strange with a:
A Gold Customer must be assigned a Personal Coach.
A rental car must be returned before 9 PM.
Here, I think, you should prefer each as the first word of the sentence.