Must is a strict requirement, no flexibility. Shall is a recommendation or a duty, you should do it. You must put gas in the car to drive it. You shall get an oil change every 6000 miles.

Well then you MUST reread RFC 2119, because your version of SHALL differs from the spec which says SHALL is equivalent to MUST and a hard requirement.

Perfectly making my point. Shall has no business being in a spec when you have unambiguous alternatives.