categories of data sets

Hello,

is it allowed to have "#user" or "#user.home-info.contact" as
values of the ref attribute of a DATA element within a policy?

If yes: In which categories are these? Why?
    (To know this is necessary for user agents providing category
    support, APPEL-compliant user agents, e.g.)

If no: Aren't these data sets? Why/why not? What is a data set then?

TIA and best regards,

Nikolaj