This translation guide is usually helpful.

https://polish2english.files.wordpress.com/2011/11/55551980-...

I'm afraid I don't know what RFC English is and neither does Google.

Huh, for me the very first non-ad result for googleing RFC English is https://en.wikipedia.org/wiki/Request_for_Comments, which is the correct citation.

For me it was to do with Rugby Football.

To be fair, here is the RFC meaning:

   SHOULD   This word, or the adjective "RECOMMENDED", mean that there
   may exist valid reasons in particular circumstances to ignore a
   particular item, but the full implications must be understood and
   carefully weighed before choosing a different course.

It means you can’t simply ignore it, and instead have to have compelling reasons to justify any deviation.

Unfortunately, in many organizations, "the library we use doesn't follow this recommendation" is a valid compelling reason. Which means that in practice "SHOULD" effectively means "WOULD BE NICE IF".