The only time I've seen a chain is in reference to a programming pin adapter plug that must stay with the aircraft when a component is changed.
Two examples are the engine FADEC units and the newer Standby (Attitude, Heading, Altimeter, Airspeed) Indicators in the cockpit.
Engine specific strapping (or coding) is on one plug that attaches to the FADEC. It is chained to the engine.
Aircraft specific data is coded to a plug that connects to the Standby Indicator. A chain connects it to the aircraft wiring harness and not the instrument.
Here's a picture of a FADEC unit. I'll have to take a pic of one (with the programming plug) on an actual engine.
Patent No.US20100167567
is a connector with foolproof structure that is protruded inside the receiving hole for blocking an insertion of an incompatible plug
So engine characteristics plug is chained to engine to prevent connecting wrong plug by fools so the chain is foolproofing structure, correct me if I'm wrong.