My description would be......
Generic:
A map developed by someone (quite possibly neither the person nor the company applying it), on a test mule car, is applied to your car and off you go. Any difference or issue between your car(s) and the car the map was developed on are not addressed.
Custom:
A starting point map is applied to you car and then tweaked real time by a tuner either on the road or on a dyno. This map takes advantage of any other modifications your engine has and can be tweaked so that the optimum performance for YOUR car is achieved whilst keep ing all relevant parameters safe.
The end result my be parameters altered so you actually run a less tuned car than a generic map, but that is all your example can safely handle or it may be the case you have a good'un and the timing can be wound on safely, running sweet air/fuel and you get that little bit extra power for your money.