The B and E bits are used to indicate the epoch of a packet. Ingress packets to the MLPPP process have an MTU, which may or may not be larger than the MRRU of the MLPPP network. The B and E bits manage the fragmentation of ingress packets when it exceeds the MRRU.
The B bit indicates the first (or beginning) packet of a fragment. The E bit indicates the last (or ending) packet of a fragment. If there is no fragmentation of the ingress packet both B and E bits are set true (=1).