The expression Rmp-Rrf is also known as Market Risk Premium. In some questions we are given the Market Risk Premium instead of Rmp. So the CAPM is written as Rrf+beta(Market Risk Premium).
The term equity risk premium is generally applied to a single stock (e.g., the difference between the return on a company’s stock and its bonds), not to the market as a whole.
I appreciate what you are saying…I am just quoting verbatim from Schweser (happen to be looking at my 2010 Level 2 books at the time). It also uses ERP in the CFA book.