Collections are also renderable, since they might contain renderable objects.
Renderable objects can have their renderable property individually enabled or disabled (enabled by default, except for pure shapes). This can be set in the object common properties dialog or through the simSetObjectSpecialProperty API function.
Additionally, renderable objects can have their renderable property overridden depending on their related model properties (if they are part of a model). Refer to the model dialog for more information.