Question

What is padding BSR and why it is needed? How ever normal BSR also can carry same information but why do we need specific padding BSR?

Answer 1

UE got the UL Grant and the padding data is larger than the size of BSR CE and the subheader (This is called Padding BSR).
There are somes cases where network send UL Grant when UE does not have any data to transmit, in this case UE transmit all 00 data or some garbage data and long padding 0s. In this situation, UE would put BSR MAC CE as part of MAC PDU and set BSR index value all 0. See one example here.