APC is something like Automatic Performance Control, and it is something Saab started fitting in some form or another since the classic 900's, allowing them to run on different octane fuels without problems, by varying the ingnition timing and - on turbo's - the boost.
You also hear people refer to DI/APC, which is what the first engine management system on Saabs is often known as, which was LH-Jetronic injection, Saab Direct Ignition, and the APC system.
Later on, they introduced their own "Trionic" system, which combined these three functions into one ECU.
You may also hear people talk about APC solenoids, or BPCV (boost pressure control valve) which is an electrically operated three way bleed valve in essence, mounted on the radiator of most turbo 9000's, which allow the ECU to control turbo pressure.
Some of the low pressure turbo cars did not have one fitted, and maybe that's where you heard of it in the context of an upgrade.
If you have an LPT car, and you want to upgrade it, it will probably involve fitting one of these. I think they come included with the upgrades from Maptun and others, if you need one.
Some LPT's have an APC solenoid, but no intercooler.