In all judgments in which the relation of a subject to the predicate is thought (if I only consider affirmative judgments, since the application to negative ones is easy) this relation is possible in two different ways. Either the predicate B belongs to the subject A as something that is (covertly) contained in this concept A; or B lies entirely outside the concept A, though to be sure it stands in connection with it. In the first case, I call the judgment analytic, in the second synthetic.
This relates to definitional mereotopology: Kant's claim is that analytic or explicative judgments are cases where, when B is predicated of A, B is a definitional part of A, and that synthetic or ampliative judgments are cases where, when B is predicated of A, B is definitionally relevant to A but not a definitional part of A, and A is not a definitional part of it. He says other things that can be taken mereotopologically, too, of course; for instance, that in (certain kinds of) synthetic judgments B is predicated of A because A and B are both parts of a whole, namely, the experiential unity combining them.