There are two basic types of spacecraft docking systems:
- Non-androgynous docking systems (examples: every U.S./Russian docking system currently in use for docking spacecraft) have two separate, distinct ports: one (the male/active/top) does all the actual work, and the other (the female/passive/bottom) just sits there doing nothing and waits to be docked to. A male port can only dock to a female port, and a female port can only dock to a male port (no gay docking ports, people).
- In contrast, androgynous docking systems (examples: APAS, Clamp-O-Tron) have only one type of port, and any port can dock to any other port.
Androgynous systems have the obvious advantage that any spacecraft equipped with a docking port can dock (external protrusions, or lack thereof, permitting) to any other spacecraft using the same docking system; there’s no need to split your fleet and equip half with male ports and half with female ports,1 or to send up an adapter any time a spacecraft equipped only with male ports has to dock to another male-port-only spacecraft or a female-port-only spacecraft has to dock to another female-port-only spacecraft. You can just grab a spacecraft out of the bucket and toss it into the right orbit, or grab a spacecraft that’s already in orbit and manhandle it into position, without having to worry about whether its docking port is compatible with the one on your other spacecraft.
Yet, despite this advantage, androgynous docking systems seem to have fallen out of favour; all current and planned U.S. and Russian docking systems are non-androgynous,2 with the only androgynous system in use today being the Chinese Docking System, used for docking the Shenzhou and Tiangong spacecraft.
Why is this?
1: Example: the Soyuz 7K-OK (the first Soyuz variant to be flown manned) was equipped to dock with another Soyuz 7K-OK, but used a non-androgynous docking system; as a result, the fleet was divided into “active” and “passive” halves, depending on whether each spacecraft was equipped with a male or a female port, and docking missions required that one active and one passive spacecraft be launched - you couldn’t just grab a couple out of the barrel at random and stick them up there.
2: For that matter, NASA seems to have shied away from docking ports of any sort, favouring berthing, even though berthing a spacecraft requires physically grabbing it with an arm on the target spacecraft (thus making it useless for attaching to a dead, incommunicado, or malfunctioning spacecraft), and unberthing takes ages and requires a person on the target spacecraft to close valves and stuff after the hatch has been shut, as detaching without having done so would depressurise the target spacecraft (thus making it useless for abandoning ship in an emergency, unless you’ve got a crewmember you really don’t like handy).