Query / Retrieval
PSYC tries to use a push approach over the Query/Retrieval model and tries to unify all asynchronous update requirements (like when your client was offline or you just installed a new one) using a generalized state synchronization approach, rather than isolated back and forth protocols.
The "?" operator
In the PSYC routing layer the ? operator is supposed to stand for something also known as _query. It requests variables from the other side. It is however not ensured, that the existence is justified.
The term is obviously also used in the SQL context, but that hardly has something to do with PSYC, indeed.