A particle can be considered with respect to its general individual function and its fully specified role in a construction. The first identifies the kind of relation it expresses; the second identifies the actual relata in a particular expression.
In “fī l-bayt” (“in the house”), the particle contributes a locative relation. A complete assertion still requires us to know what is in the house. The particle can therefore have conventional significance without constituting an independent proposition.
The key point is that inability to stand alone as a complete statement does not entail absence of an assigned meaning.