[Dev-luatex] user_defined whatsits

Hi, once in a while I find surprises in the luatex reference manual, such as the user_defined whatsits (great!). 1) Is the user_id field completely up to me? So I can use it somewhat as an attribute? 2) Will the user_defined whatsit stay in LuaTeX (I know, nothing is fixed, but the developers might have a tendency) 3) am I dreaming or is this really real? Patrick