On Aug 26 2012, Rolf Rabenseifner wrote:

About 1.: Yes, agreed. Always must and should are used as synonyms in the scope of standards. To use the same words in both sentences is clarifying that there is no relevant difference in the meaning.

Sorry, but definitely not in ISO standards! From ISO Directives Part 2: Table H.1 - Requirement Verbal form Equivalent expressions shall is to is required to it is required that has to only ... is permitted it is necessary shall not is not allowed [permitted] [acceptable] [permissible] is required to be not is required that ... be not is not to be Do not use "must" as an alternative for "shall". (This will avoid any confusion between the requirements of a document and external statutory obligations.) Do not use "may not" instead of "shall not" to express a prohibition. Table H.2 - Recommendation Verbal form Equivalent expressions should it is recommended that ought to should not it is not recommended that ought not to [End of extract] However, in non-ISO standards (and fairly often even in them!, "must" is used as a synonym for "shall". It is definitely never a synonym for "should". Regards, Nick Maclaren.