This topology (shown below) allows for multiple simultaneous paths, and allows inputs to share outputs. (Input signals are combined into a shared output). Full-Fan-In matrices are the complement to the Full-Fan-Out topologies, and are typically used at Uplink or Transmission sites.
Example: Input 2 is routed to Output 1, and Input 3 is also routed to Output 1. At this point in time, Output 1 (or any other output) can be routed to other available inputs.