What exactly is "dominant" and "recessive"?
Dominant is 0.  Recessive is 1.  Dominant applies to 0 because if two arbitration ID's are being transmitted at the same time and the first 4 bits are the same and the fifth is 0 for one of them and 1 for the other, the ID with the 0 will end up being transmitted.  Transmission of the message with the larger arbitration ID will be tried again after the other message is done.