The combinations with BGF2 set are reserved.[1]:7–8 Bit 74 ("Binary group flag 1") was previously unassigned, but is used to indicate that the time code is synchronized to an external clock. If zero, the timecode start time is arbitrary. The checksum in bits 82–89 is a simple bytewise XOR of the previous 82 bits (including the sync bits, so bit 82 is the XOR of bits 74, 66, ..., 2), which can be described as a CRC with generator polynomial x8+1.