Generally there are only a few ROM calibrations for one physical ECU, but almost every new model year comes with a slight revision in hardware. There are exceptions. Say, a 2002 WRX may have one physical ECU and three compatible factory calibrations. If you flashed a 2004 WRX image to it, it would probably brick the ECU.
For Subaru it goes by CAL ID. (i.e. A2ZSJ500J, A4SGC00C, etc). To be certain to avoid problems (like frying an ECU) always read out the ROM from an ECU, tune that, and flash it back. At least don't flash a different CAL ID than you found on the car before you started unless you are sure it is compatible or you'll be a sad panda.
I think Enguinity will pop up and tell you if there is a newer compatible, safe calibration that you could choose to use. There may not be good information on what exactly is "better" about it, other that the factory deemed it a necessary update. It could be a negligible difference, especially if you're retuning the car, could be emissions stuff, etc.
Scoobypedia has a compatibility list here:
http://www.scoobypedia.co.uk/index.php/ ... bilityList
