I’m not an expert in CLP (Constraint Logic Programming). As I understand it, only “posting” the constraints and calling the labeling predicate has well defined semantics. If you don’t call labeling you may end up with a set of conflicting constraints, a concrete answer or a legitimate set of constraints. Documentation, source or trying are FAIK your only answers.
Note that this is somewhat related to clause indexing and choice points discussed here before: there are no guarantees. Clause indexing typically reduces search and choice points,but whether or not it makes a call deterministic remains largely undefined.