PIM components can be set to either sparse mode or dense mode. Sparse mode is the default.
Sparse-mode routing protocols use shared trees. In a shared tree, sources forward multicast datagrams to a directly connected router, the designated router. The designated router encapsulates the datagram and unicasts it to an assigned RP router, which then forwards the datagram to members of multicast groups.
Dense-mode protocols are data driven, where multicast sources start sending multicast data packets, and receivers join if they want data packets or prune themselves.