YAP documentation it's a bit more explicit, I cite from there :
multifile P [ISO] Instructs the compiler about the declaration of a predicate P in more than one file. It must appear in the first of the loaded files where the predicate is declared, and before declaration of any of its clauses. Multifile declarations affect reconsult/1 and compile/1: when a multifile predicate is reconsulted, only the clauses from the same file are removed.
Still, a clarification about module interaction would be welcome.