The C pin is control. Later model subarus, generally obd2 + use 3 wire plugs do be able to switch the alternator on and off.
I am pretty sure you can use any alternator that fits on the plug. It just means the computer wont be controlling it so it will run all the time as your standard one does. Don't quote me on this, this is why I recommend a gen 3 outback alternator. 95A is a good upgrade without putting to much load on the engine., and I know for a fact it works fine.
Another note on alternators is you cant go from 3 wire two two wire alternators/ wiring on a car with factory 3 wire. The car will throw a check engine light. Was one aspect I had to address when putting an 04 wrx engine with 3 wire into my 2 wire gen 3 outback, I couldn't use the standard alternator wiring from the fuse box.
