The "WRX" version supplies a regulated voltage to a pin on the OBD connector which supplies the ECU with a voltage to reprogram it's flash memory. With the appropriate software, you can then use this to reflash your ECU. This pinout is appropriate for 2002-2005 WRX's and USDM STI's and FXT's.
The "generic" version is basically the "WRX" version with this pin removed. If you have a different model Subaru that doesn't use the same programming pin, or a non Subaru car that still has a ISO-9141-2 interface, you can use this version to do things like OBD2 and Subaru SSM logging without the ability to reflash.
Both versions are the same price because I basically only manufacture the "WRX" version and then have to take time to remove parts for the "generic" version. I thought it was worth offering though.