The IE RRC-TransactionIdentifier is used, together with the message type, for the identification of an RRC procedure (transaction).
RRC-TransactionIdentifier ::= INTEGER (0..3)
UE shall set the rrc-TransactionIdentifier in the response message, if included, to the same value as included in the message received from E-UTRAN that triggered the response message
source: RRC spec 36.331 (section 6.3.6)