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.
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.