The TX and RX Exchange fields are used for logging exchanges for QSOs made during a contest. The TX (transmitted) exchange is the exchange you sent to the other station. The RX (received) exchange is the exchange you received from the other station.
The exact details of the exchanges depend on the contest you're operating in. See the contest rules for information about exactly what the exchanges should contain.
Importantly, if you're not using Aether to log contest QSOs, you can simply ignore these fields. You can also log whatever information you'd like in them.
If you are using Aether to log contest QSOs, make sure you feel in these fields, as they are used when exporting the Cabrillo file you will submit to the contest sponsor.