When the de jure coreference relation exists between two singular terms, A and B, within a statement or discourse, the coreference of the two terms is presupposed - taken for granted by anyone who understands the discourse. Campbell's inference then becomes legitimate: from Focus on is F and B is G, we can conclude that there is an entity x that is both F and G. Since the mental file associated with the two singular terms is the same, the predicates associated with these terms within each premise feed the same mental file. And what's special about a mental file is that all the properties listed in it are held to be properties of one and the same entity, namely that to which the mental file refers (its reference). Campbell's inference is therefore legitimate. When the two singular terms are associated with distinct mental files, which may refer to different objects as well as to the same object, co-reference is not guaranteed and the inference becomes illegitimate.
If Campbell's inference is valid because the two terms A and B are associated with the same mental file, then we can use Campbell's inference as a criterion for determining whether two terms are, or are not, associated with the same mental file. In the statements A is F and B is G, the terms A and B will be said to be associated with the same mental file in all cases where Campbell's inference is legitimate. If it is not, then the two terms are associated with separate mental files.
