My best guess, and this is only a guess, is that they may be used to signify modified versions of API trim 2 and API Trim 16.
API Trim 2 is both seats and stem as 304 stainless steel material. API trim "2H" is possible the same except 304H stainless(high carbon 304) as the material
API Trim 16 is both seats stellite (Hard faced) and stem as 316 stainless steel material. API trim "16H" is the same except 316H stainless (high carbon 316) as the material
one clue is would be the body material if it's something like cast CF10 or CF10M or forged F304H or F316H. another is if the valve is in service over 1000°F