Object instances are the actual objects you see in a game: an instance of an object type. For example, if there are four TrollEnemys in a layout, those are four instances of the TrollEnemy object type.
It is instances which have a position, angle and size in the layout. Object types do not have these properties - they simply define a 'class' of object.
Instances can be created at runtime in the event system (typically by the System Create object action and the Sprite Spawn an object action). They can also be pre-arranged in layouts with the Layout View to design levels, menus and title screens.
Selecting an instance in the Layout View shows its properties in the Properties Bar. These are a mix of properties in common with all (or most) objects, and plugin-specific properties. The common properties are described below, and plugin-specific properties are described for each plugin in the Plugin reference.
The following properties are common to all or most objects, depending on their capabilities.