The difference is that there are 2 types : SR1 and others. The software with a B has both the CPM and IOM functionality in it and is seen in SR1. The TiMOS-C is the CPM image used for routers like SR12 which have different software images for CPM's and IOM's. Similarly there will be a TiMOS-I for IOM images.
B - SR1 image
C - CPM image for SR4/12
P.S: I am pretty new to this myself