Encore, also called "RangeMax" or "BroadRange" is something Broadcom invented to increase receiver sensitivity.
Encore is available for OFDM only, hence in A and G PHYs. It was introduced to A PHYs with PHY revision 6 and to G PHYs with PHY revision 7, but is not available on all PHYs with such revisions. To determine whether it is available check the encore register (A PHY Register 0x49) for bit 0x200 which indicates availability.
The encore functionality itself is pretty much implicit, PHY programming differences are minute.