Yesterday SpaceX and NASA successfully docked the Dragon 2 capsule for CRS-21 to the International Space Station in part of the phase 2 Commercial Resupply Services contract with NASA.
CRS-21 docked to International Docking Adapter 3 (IDA-3) which is on top of the Harmony Module. This is the first time IDA-3 has been used to be docked with a visiting spacecraft; all previous spacecraft have used IDA-2 which is on front of Harmony. These IDAs are attached to a piece called the Pressurized Mating Adaptor which was used by the Space Shuttle to dock to the station. These adaptors were attached over the past several years to allow for a standard docking system to be used across commercial vehicles.