It's a seal accessory — not a housing or terminal — designed to fit into a compatible panel mount housing to keep moisture and dust out of the connection. The yellow colour makes it easy to spot during assembly or rework, which helps when you're pulling a harness apart in the field and need to confirm the seal is seated.
Mating and Fit — What It Seals
This seal is intended for use with a panel mount housing — the Nector M series housing that accepts the 3-position seal. The silicone rubber material gives it the compliance to compress into the housing cavity and form a reliable environmental seal. If you're rebuilding a connector on site, check that the housing you have is a Nector M panel mount type with 3 cavities; the seal drops in before the contacts and backshell go on.