Sixty-four bits, one per frame, counting up from the starting sequence number. Set means received.
Eight octets doing the work of sixty-four separate frames. A gap in the bitmap is a retransmission request without any request being sent — the transmitter simply resends what is not marked, which is why a capture full of Retry frames alongside healthy Block Acks means loss in one direction only.
Compressed Block Ack MPDU > Block Ack Bitmap