This Avid Post Mount Disc Brake Spacer Kit 180mm Rotor includes standard and CPS spacers, mounting bolts and washers to fit between post-mount calipers and post-style caliper mounts. This spacer kit will fit 180mm front rotors and 160mm rear rotors on appropriate frames. Some forks have post mounts that will natively fit 180mm rotors, and some frames have post mounts that will natively fit 160mm rotors, so double-check before ordering.
Specifically, this adapter kit will add 20mm to your fork's/frame's post mounts. If your fork has post mounts for a 160mm rotor, this adapter will allow a 180mm rotor to fit properly. A frame with 140mm post mounts will accept a 160mm rotor with this adapter.