This MAC header can be only sent by the anchor. It's used in data messages during backchannel transactions.
Structure of BD MAC header:
MAC header type identifier - 0xBC - Backchannel poll message
MAC address of the tag
The sequence number of the blink.
The version of backchannel protocol
BC transaction number - this number is generated by the server for every BC transaction.
Options that determine if the tag should send immediate ACK, and how many ACK tags should send after the successful reception of the BD message.
Lowes bite - if 1 - immediate ack should be sent after receiving this BD message.
Bite 4:1 - number of ACK, that should be sent by tag after successful reception of this BD message. <0;15>
Other bites are unused.