The Zen of Law Writing ~ “Must vs. Shall”

Law writers use the word "shall" to denote a duty imposed on a person or an entity.

Law writers use the word "must" to connote a prerequisite or condition precedent that requires meeting certain conditions, consisting of certain components, or possessing certain characteristics.

Law writers sometimes prefer "must" if the word is used in a sentence in which the subject is an inanimate object rather than a person or entity on which a duty can be imposed. ~ Example: "A grievance must be filed within 90 days of the adverse action."

At times, either "shall" or "must" is correct. So, a law writer will consider the context and desired point of emphasis. ~ Example 1: "Notice must be given in writing." versus Example 2: "A review shall be conducted within 45 days of the filing of a formal request for review." ~ Consider that Example 1 relates to a required characteristic of the notice, while Example 2 relates to a person with the duty to conduct the review.

Law writers can often avoid the entire issue by writing the "facts and nothing but the facts" when appropriate. Example: "The successful candidate qualifies for the position by scoring the highest on the test."

Source:
http://lawist.blogspot.com/2009/12/zen-of-law-writing-must-vs-shall.html