What is this part called? I built a new YT Capra yesterday, and it had a part I had not seen before. It was not mentioned anywhere in the instructions, but it was needed to get the rear mech the correct distance from the rear mech hanger.

See in the image below, in the red box. It is a kind of spacer between the rear mech and the rear mech hanger. If I didn't use this, then the mech would hit the frame.

I haven't got a picture of it on its own. It was a small ring with a notch on it. That notch lined up with the notch on the hanger to give it an extra couple of cm. I'm wondering if this is something YT have built especially for this edition (it is not on older ones), and it doesn't exist anywhere else?
That's part of the derailer. Most SRAM derailers have a piece like that. They're not really supposed to come off and float freely but sometimes they do anyway. There's not a specific name for it, but if you wanted to buy a new one for this X1 rear derailer, it's part of SRAM pn# 11.7518.014.000, "XX1/X01/X1 Rear Derailleur B-Bolt and Limit Screw Kit".