TMGI stands for "Temporary Mobile Group Identity". While establishing each eMBMS session, BM-SC server provides provides a unique identifier i.e. TMGI to identify a MBMS session within the PLMN.
TMGI is a combination of "PLMN" + "Service Id" . As usual, PLMN is encoded by using 3 octets and service id is also 3 bytes long.
SessionId is an optional information which may be provided by BM-SC server at the time of session establishment.