An exchange is the name of the current encryption state to one remote endpoint, a pairing between the local endpoint's private key and the remote endpoint's public key.
When a new exchange is created it will generate a unique public ROUTING TOKEN value for that current instance, any incoming encrypted channel packets with that matching value can then be delivered to the appropriate exchange.
The exchange is used to generate and process all messages, handshakes, and channels. It can only generate channel packets after it has processed at least one handshake from the remote endpoint to set up the required synchronous encryption state.
An exchange internally keeps track of the current/best at value for the handshakes, validates incoming channel IDs, and generates new outgoing channel IDs.