Not necessarily sexual, but as a "proof of belonging" thing. James gives him a collar that he can choose to wear whenever and wherever he wants, just to remind himself that he is wanted and cherished and belongs to someone in some way. It's a soft, buttery leather, pretty plain. But when Booker turns his head while wearing it, the edges press into his neck just a little, and he's reminded that he belongs to, and with, James, for as long as they can.

Could also be sexual, ngl, but doesn't have to be. It's just been in my brain for weeks.