In simple words:
CircID serves the purpose to separate the circuits = onion 3-node chains. As it described in design - I'm citing: The header includes a circuit identifier (circID) that
specifies which circuit the cell refers to (many circuits can
be multiplexed over the single TLS connection), and a com-
mand to describe what to do with the cell’s payload.
Yes. You will probably want to see these two blog posts: