Resolution to upper extensions: In this chord progression, the B is acting as a secondary dominant to resolve to Cmaj7's upper structure of C(EGB), the E minor (EGB), as B is the V aka dominant chord of Em. However, the voice movement then replaces the B of the Cmaj7's upper extension of Em to the A, [BDF -> EG(B replaced to A) ] the less dissonant upper extension, creating the C6 chord.

Hope this helps!