Referencing Category Affinity through user.categoryId as an mbox parameter?

I understand that category affinity is contingent on having a value set to the parameter 'user.categoryId'. I was wondering if it were possible to reference user.categoryId by itself as an mbox parameter in my audience rules, as opposed to using the standalone category affinity algorithm? Suppose user.categoryId is associated with the category of my product, can I build an audience whose categoryId matches a specific value?