MSU is a signaling unit which will have the following format.
Flag: 8 bits
SIF: (Signaling Info: MTP3 sent buffer)3 x 8 to 272 x 8 bits
SIO: 8 bits
LI: (Length Indicator) 6 bits
FIB:(Forward Indicator Bit)1 bit
FSN:(Forward Sequence Number) 7 bits
BIB:(Backward Indicator Bit) 1 bit
BSN:7 (Back-ward sequence number) bits
And this will be with MTP-2, only the SIF ( Signaling Information ) field is used by MTP-3. I mean the MTP-3 message embedded as SIF.
And MTP-3 is take care of distributing to respective application ( ISUP, SCCP etc).
Hope the above said was helpfull.