When Alice and Bob use Keychat to chat, they can both unidirectionally use two relays, A and B, simultaneously.
Alice uses Relay A to send messages to Bob, and Bob uses Relay B to send messages to Alice. Alice receives messages from Relay B, and Bob receives messages from Relay A.
Thus, Relay A is Alice's sending relay and Bob's receiving relay. Relay B is Bob's sending relay and Alice's receiving relay.
This is the flexibility of the relay model.