DETEX REL-DTU-8 works with the standard Reliant reader.
This comes with the standard Reliant Kit - reader, holster, software, stations..
You would normally purchase this if you damaged or lost the one that came with your system, or were adding a new reader to an old style system.
Around 2014, Detex replaced their older readers with the more compact and energy efficient readers. This downloader (DTU, Digital Transfer Unit), plugs in to the old system and allows you to use the new readers with the old system's software, saving you the cost of upgrading everything.