The black thermoplastic, glass-filled polyester body carries a single row of 4 cavities and uses a latch lock for retention against the mating plug. This is the key-X polarized version, which means the housing geometry prevents cross-mating with other key codes in the Dynamic series, a useful feature when multiple similar connectors share a harness.
The key-X designation on this Dynamic D-4200 housing refers to the physical keying of the mating interface. TE uses a letter-based keying system (X, Y, Z) on these connectors to prevent mismating between different signal or power circuits in the same harness bundle. A key-X receptacle will only accept a key-X plug — it will not mate with a key-Y or key-Z counterpart. For a harness builder, this means you can run multiple Dynamic connectors on the same cable bundle without risk of a wrong connection during field service. The keying is molded into the housing shroud, so there is no loose insert to lose.
The termination method is crimp, which means the terminal is attached to the wire with a crimp tool before being inserted into the housing cavity. The mating plug is a male header from the same Dynamic D-4200 series with matching key-X polarization and the same 4-position count.