This is the purple plastic "U" shaped pin-retainer clip that slides into the ECM connectors on the 2006-2010 LBZ/LMM Duramax (also fits the 2006 LLY trucks).
"C1" is the larger 96-pin ECM connector
"C2" is the smaller 56-pin ECM connector
The "C2" connector has two separate retainer clips, see picture
It is very common to either lose or break this small clip when disassembling the ECM connectors to install an EFILive DSP-5 switch or high-idle kit. BT DieselWorks has you covered with a genuine Bosch-OEM replacement part. This clip is NOT serviced separately from GM; if you go to the dealer, they will only sell you an entire replacement connector. Why pay the dealer $75+ for the entire connector when all you need is the small plastic clip?