A = I know the customer, they own this number.
B = I know the customer, can't confirm they own this number.
C = I'm sending this call out, but I know nothing of the customer or this number.
Carriers sometimes don't want to receive anything other than A, because its probably useless to them. By that I mean, if it's not A, they don't want to be sent the attestation level or identity header at all.