what are diffirence between PIM-SM and PIM-DM?

PIM-DM: Small-scale LANs where multicast group members are distributed densely. Using the flood-prune mechanism, PIM-DM creates and maintains a unidirectional and loop-free SPT connecting a multicast source and group members.
PIM-SM(ASM model): Large-scale network where multicast group members are distributed sparsely. A****T is set up when receivers join a multicast group. PIM-SM needs to maintain an RP, set up an RPT, and register a multicast source.
PIM-SM(SSM model): Scenarios where user hosts know the exact positions of multicast sources in advance and can specify the sources from which they want to receive data before they join multicast groups. PIM-SSM does not need to maintain an RP, set up an RPT, or register a multicast source.