This is a List<GameItemObject> which has some helpful methods for managing the data at runtime. You can have multiple Lists of this type on an object, to hold various lists of GameItemObject data, such as:
public GameItemObjectList inventory = new GameItemObjectList();
public GameItemObjectList equipment = new GameItemObjectList();
Both GameModulesActor and GameModulesInventoryActor have an inventory and equipment as shown above, as those are a very common use case for Actor objects.
If you inherit from GameModulesActor or GameModulesInventoryActor, you can choose to use both, or not, or even create your own additional GameItemObjectList variables.