My guess would be it's the parts of the game that are generally useful and polished enough to be used as the base for other games (or non-game applications).
Games often have parts that don't quite belong to the engine layer, but also not strictly to this specific game. It might be small adhoc helper modules, glue code to other 3rd party libraries or similar random stuff.
Games often have parts that don't quite belong to the engine layer, but also not strictly to this specific game. It might be small adhoc helper modules, glue code to other 3rd party libraries or similar random stuff.