Can enum/set be optional?

[OleksandrKvl]

Hi, in Euronext MDG schema there are bitset (<field id="5" name="strategyAuthorized" presence="optional" type="StrategyAuthorized_set"/>) and enum (<field id="3" name="eMM" presence="optional" type="EMM_enum"/>) fields marked as optional, is it legal? My understanding is that field with a non-built-in encoding always inherits and cannot change presence of its encoding (if only to constant). I can hardly imagine how this can be legal because 255 can be a valid enumerator and bitset can have all bits set which results in a value 255 for a set encoded by uint8. Can you confirm that it's not allowed?