3.2.a Troubleshoot reverse path forwarding

Strict RPF—Using the rx keyword, the router checks to see if the matching route uses an  outgoing interface that is the same interface on which the packet was received. If not, the  packet is discarded.

clearly the best definition of rpf i have come across

Loose RPF—Using the any keyword, the router checks for any route that can be used to
reach the source IP address.