To configure Carrier Aggregation, Network send Secondary Cell (SCell) configuration to only those UEs which are release 10 and support Carrier Aggregation. Otherwise, UE wouldn't be able to process the SCell configuration from the network and abnormal things will happen. Also, network should be aware of for which all band combinations CA can be configured i.e. which band's cell should be used for SCell.
Now question here is, how would network know if UE is supporting Carrier Aggregation or not and if supporting, which all band combinations can be used for CA?? The answer is UECapabilityInformation Message. During Lte Registration procedure, UE reports detailed Carrier Aggregation (CA) capability in UECapabilityInfomration Message. Please refer the following MSC:
3GPP References
As per spec 36,331, UECapabilityInformation has following release 10 IEs:
