Protocol Configuration Options (PCOs) are used to exchange information between UE and PGW.
As per your ask, UE has request for IP MTU from network and network will reply to UE with the information asked by UE. There are so many elements/ information exchange between UE and PGW. For example: UE preference for IP address allocation (either IPv4 or IPv6). There is a huge list of protocol container Id.
As per my knowledge, UE asked about IPv4 MTU size in advance to avoid possible IP fragmentation at PGW for the uplink data.
MME and SGW transparently passes PCO information to PGW receives from UE. And also in reverse direction.
Please refer 3GPP TS 24.008 for more detailed information on all defined PCO values.