CNC machined from billet aluminum, the Hope Brake Caliper Adapter is a Post Mount extender that mounts onto your fork to accommodate a 220 mm rotor.
There are 2 sizes available: +17 mm and +20 mm. Most DH forks (Fox, Marzocchi, Öhlins, RockShox, Manitou…) are at the 203 mm Post Mount standard and need the +17 mm adapter. This is also the required size to adapt on the proprietary DNM mounting system (Sur Ron, Talaria).
A few forks (notably the EXT Ferro) follow the more recent metric standard for 200 mm discs, in this case, you need the +20 mm adapter.