Bearer Control Mode is basically used to tell bearer creation can happen only from UE side or both from UE and N/W side.
In LTE it is always UE/NE. Bearer creation can happen both from N/W side when PCRF sends RAR message with Charging-Rule-Install. From UE side bearer creation request will come when MME will send Bearer Resource Command
In case of legacy network Bearer Control Mode depends on 2 factor if the SGSN and or UE supports UE/NW mode. If either of them does not support bearer creation from N/W side Bearer Control Mide is sent to PCRF from PGW as UE only.
In case a UE handovers from LTE to 3GPP-GRPS in that case based on SGSN capability BCM could be changed to UE only.
Hope it helps.