This MAC header type is sent by tag after the backchannel data message from the anchor is successfully received.
Structure of BA MAC header:
MAC header type identifier - 0xBA for acknowledgment
MAC address of the tag
Sequence number of the blink.
Version of backchannel protocol
Acknowledgment number - this number is generated by server for every BC transaction. Tag do not interpret it. It only put the last received transaction number into ACK header.