Robert Brandom seems to have a very distinct notion of implicitness when he says things like:
For while that vocabulary is not itself descriptive vocabulary, its use is implicit in the use of ordinary descriptive vocabulary. 
By "that vocabulary" he is talking about nondescriptive vocabulary that plays a "categorial metalinguistic expressive" role. What I take from this is that he has the idea that there is some vocabulary (nondescriptive), the use of which I already and implicitly know just by knowing how to use some other vocabulary (descriptive). But how am I to understand this very notion of implicitness? Also and very relatedly, what sort of knowledge does he have in mind?
 Brandom: From Empiricism to Expressivism, Cambridge: Harvard University Press, 2015, p. 90.
*Note on the source: The ideas at work here must be very similar or identical to those in his Making it explicit, but unfortunately I haven't read it yet.