Earlier quoted context omitted.
In general "should" is a recommendation, not a requirement. "shall" indicates a requirement "should" indicates a recommendation "may" is used to indicate that something is permitted "can" is used to indicate that something is possible, for example, that an organization or individual is able to do something https://www.iso.org/foreword-supplementary-information.html https://www.ietf.org/rfc/rfc2119.txt Additionally, y…
If you don't mind me asking, what's the point of "should"? Usually anything that is not a hard requirement is promptly ignored, so I'm not clear why is time devoted to create "should" statements.
This is advantageous to the agency if they can get away with it because new rulemaking involves a bunch of extra, lengthy process under the Administrative Procedures Act (they have to publish a bunch of drafts and collect public comments on them, then address any substantive comments they receive, etc.).