For DFARS users, the DoD 5025.1-M (DoD Directives System Procedures) states the use of helping verbs in DoD Directives, Instructions and Publications:
Helping Verbs - Degree of Restriction
Must, shall - Action is mandatory.
Should - Action is required, unless justifiable reason exists for not taking action.
May, can - Action is optional.
Will - Is not restrictive; applies only to a statement of future condition or an expression of time. Do not use in place of "shall."