If you create an object while in an active render layer (that is, a layer other than the master layer is set as visible), a collection named _untitled_ is automatically created that contains this object.
Additional objects that you create while in the render layer are also added to the _untitled_collection.
You can rename the _untitled_ collection. However, after you rename this collection, if you create additional objects while in the active render layer, a new _untitled_ collection will be created again.
Additional _untitled_ collections (created in other render layers) are named _untitled_1, _untitled_2, and so forth.