We might also grant that for judgment to be possible, the judger must be able to assign the components of the judgment to a consciousness of the self. This is the analytic proposition that stands behind all concepts of combination. The assignment to all the representations of a judgment to myself is an act of synthesis, so the representation of the unity of self-consciousness is synthetic in a sense. In Section 17, Kant states that the conditions which make the unity of apperception possible also make the unity of intuition possible. I suggest that it is categories which are the conditions of all synthesis, be it that of intuition or of self-consciousness.
Categories are functions of unity, principles of combination.
It may be that categories can unify intuitions only insofar as they are able to bring unity to self-consciousness. Sometimes it seems to me that Kant's use of the notion of the unity of apperception contains a modal fallacy. That is, the ability to bring representations to the unity of consciousness is a necessary condtion of all combination. But it is not a principle at all, but the outcome of a process of synthesis, just as is the unity of intuition. At any rate, by the end of Section 20, we are told that categories are conditions for the unity of all intuition.