Due to re-establishment at lower layers. Some PDCP packet can be lost . So those bearers which are mapped to RLC AM, receiver entity sends PDCP Control PDU for PDCP status report so transmitter can re-transmit those missing packets towards receiver.
PDCP Control PDU for PCDP Status report has mainly "FMS field". FMS stands for first missing PDCP SN. And a bitmap which indicates that what are the packets need to re-transmit and what are not.