We got the TB block and we decoded those TB blocks, which we received from PDSCH in form of MAC PDU, and this MAC PDU consist one or more MAC subPDUs and optional padding & the presence and length of padding are based on the available TB size and the size of MacPDUs.
And each mac subPDU consist (BI,RAPID,RAR).
Your device may be received backoff indicator with MAC PDU, BI basically six bits which we use to set the value 5 ms to 1920 ms, it means you can enable the BI at gnB, when this happens, gnB will tell UE to back off for a period of time before attempting random access gain.
If no subMAC PDU, BI should be 0 ms.
RAPID we will use only in special cases.