In general "should" is a recommendation, not a requirement.
"shall" indicates a requirement
"should" indicates a recommendation
"may" is used to indicate that something is permitted
"can" is used to indicate that something is possible, for example, that an organization or individual is able to do something
If you don't mind me asking, what's the point of "should"? Usually anything that is not a hard requirement is promptly ignored, so I'm not clear why is time devoted to create "should" statements.
The way they've framed this is not that it's a new rule, but rather, a statement as to how they intend to enforce the existing rules that are already on the books, and a "recommendation" to regulated entities as to what actions they should/shouldn't take in order to not suffer negative enforcement consequences (in other words, it's not "the rule is now that you must do this," but rather "just FYI, our interpretation of current rules/statutes/whatever is that behavior X is already prohibited, so if you don't want to get in trouble with us for failing to comply, you really ought to do this").
This is advantageous to the agency if they can get away with it because new rulemaking involves a bunch of extra, lengthy process under the Administrative Procedures Act (they have to publish a bunch of drafts and collect public comments on them, then address any substantive comments they receive, etc.).
Should allows for someone who has a real exception that we can't even think of to explain themselves. Maybe such a thing doesn't exist, I can't think of it. But just in case you have an out.
Those words actually differ in these sorts of documents but are used as "terms of art".
Shall is a mandatory requirement.
Should implies a goal and is non-mandatory.
Must is not often used, since it really doesn't seem different from Shall.
Everyone is responding with quotes from IETF and ISO documents. But this is a legal context, and it is not necessarily the case that they have the same technical meaning. I too wonder what the answer to your question is.
"Should" means that there are scenarios where doing something is not necessary, and therefore really does not constitute a hard requirement. "Shall" means that you are inherently required to do something; it is much closer (if not identical) in meaning to "must". "Should" is the subjunctive mood; there is an implied "if" somewhere in there: You should do this if blah blah blah, I would do this if blah blah blah, etc.
There are probably some subtle connotational differences between "shall" and "must" that the average reader would not care about (and which I don't feel like figuring out)
My two cents. "Shall" and "must" seem identical at face value, but functionally they are quite different.
Compare "You shall not pass this spot!" with "You must not pass this spot!" The former is a straightforward command. This is what must (not) be done, period. The latter has a strong additional implication that "If you insist upon passing the spot, something bad will happen to someone."
If you're at a party drunkenly acting out and your host comes to you and says, "You shall leave right this instant" there isn't any choice in the matter. It's a forgone conclusion that you will leave, end of discussion. "You must leave right this instant" sounds like we want you to leave, and if you don't, there will assuredly be regrettable, albeit unstated consequences.
So, "must" tends to have an implication that a detrimental outcome will definitely happen if the exhortation is disregarded. Yet statutes and contracts usually use "shall." I think this is largely traditional. One must obey the King's Law, not because of any penalty, but because it is the King's Law. But it also has an operational advantage -- it separates the command from the implication of assured penalty. One might think that implying an assured penalty is better, so what gives?
Well, a typical statute might read, "All documents shall be submitted in black ink. All submissions shall total no more than 10 pages." If the shalls were replaced with musts, it would sound as though any invalid submission will be rejected out of hand. But just leaving them as pure "shall" commands gives legislators flexibility in crafting a penalty as weak or strong as desired. E.g. "Failure to adhere to any of the preceding requirements may result in the submission being rejected." We might punish you, or maybe not. Or perhaps, "failure to comply shall result in any pages after the tenth page deemed defective and omitted from consideration." There's a consequence, but as long as you're ok with it, go right ahead. Sometimes legislative sessions wind up deleting the passage with the penalty and then you're left with a "shall" with no penalty at all.
