Improve `VARS` / `OPTIONAL_VARS` definitions and distinctions

[fmigneault]

As shown in the discussion: Originally posted by @fmigneault in #621 (comment)
(and many other issues/discussions encountered before)

There are a lot of inconsistencies regarding the interpretation and expected behaviour of VARS / OPTIONAL_VARS. They should be better defined to avoid this confusion, since the current definitions leave room for interpretation.

Once definitions are updated, variables that do not respect them should be adjusted accordingly. If the result ends up being that most variables move from OPTIONAL_VARS -> VARS, then there could be some further evaluation of the purpose to even distinguish them.