We call "must" and "must not" words of obligation. "Must" is the only word that imposes a legal obligation on your readers to tell them something is mandatory. Also, "must not" are the only words you can use to say something is prohibited. Who says so and why?

Nearly every jurisdiction has held that the word "shall" is confusing because it can also mean "may, will or must." Legal reference books like the Federal Rules of Civil Procedure no longer use the word "shall." Even the Supreme Court ruled that when the word "shall" appears in statutes, it means "may."

Bryan Garner, the legal writing scholar and editor of Black’s Law Dictionary wrote that "In most legal instruments, shall violates the presumption of consistency…which is why shall is among the most heavily litigated words in the English language."