Gen IV harness. What's this plug?

[bigmoe]

I've got my motor ready to go into my truck. Thought it would be easier to put the harness on before it's in the engine bay. It's a Gen IV 08 5.3. I can't figure out what the purple plug is for on the left of the picture. It's at the same end as the big plugs for the ECM. It's on a fairly long lead. I assume it's goes somewhere off the engine. (The picture is not my harness but a pic of PSI's harness.) Not seen on the right side is the OBD II plug.