Need help understanding something in diameter_service.erl - why is the 'relay' application excluded?

I’m having difficulties with these lines

My use case is rather simple: I am a “relay” application and have a packet I want to route after I’ve “manipulated” it… in short I’m a Diameter Proxy.

In diameter_service, the pick_peer function in there down the line tries to find_outgoing_app to route the packet towards the relative transport.

I don’t understand why the 'relay' application is excluded from the game by that case