What is "cover-to-cover indexing"?

I came across this term reading my lecture notes; I can't find it on the internet.

My notes say the following:

Given a schema R(A,B) and a query of the form "A=a", or "B=b", or "(A=a AND B=b)". Two indices over "A" and "A,B" provide a cover-to-cover indexing of the schema.

If the schema contains N attributes, then the binomial (N, N/2) is the number of indices necessary to provide a cover-to-cover indexing.

Can someone shed some light on this topic? What is cover-to-cover indexing?