I found some more info in the CTAP2.0 spec that I had previously missed:
"8.1.3. Concurrency and channels:
A channel identifier is allocated by the FIDO authenticator to ensure its system-wide uniqueness. The actual algorithm for generation of channel identifiers is vendor specific and not defined by this specification."
Can this just be a sequentially increasing ID or serial number? What happens if there are multiple devices with the same CID?