Easiest way I have thought about is the following:
Lynx RX - TX - either takes DMX from the dongle via the CAT5 Cable and TX (transmits) it out to your DMX network or RX (receives) DMX information from ANOTHER TX and puts that information out via CAT 5 depending on the jumper selection you have chosen.
Lynx EX - RX - Is basically a RX (receiver) only that is called a "daughterboard" since it requires another PCB to operate. In this case the Lynx EX-RX is used right now to make the Lynx Express, Lynx Aether, and Lynx SSR wireless.
Real world what does this mean.
Well you can have one Lynx RX - TX at a Lynx express in your yard and then use CAT5 to another Lynx Expresses in your network. This essential makes the first jump from your computer to your first Lynx express wireless (not having you find some interesting way to feed CAT5 out of a window or drill a hole). This example would require 2 of the RX / TX's. One to act as the TX and the other RX.
The other example is having on RX - TX to TX (Transmit) the data and have everything in your network (Expresses, Aethers, SSR) utilizing the cheaper EX-RX board to receive the information, making your whole network wireless.
Hope that helps as that is the way I grasped it around my mind to make sense.