If you create an object while in an active render layer (that is, a layer other than the scene 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.
Disable Options > Enable untitled collections when new objects are created to avoid creating an untitled collection when you add new objects to the active render layer.
You can disable this option at any time, even if there are untitled collections in your scene, and even if there is an untitled collection in your current render layer. In the latter case, any additional new objects that you create will not be added to the existing untitled collection.
This setting is saved with your preferences in the option variable renderSetup_useUntitledCollections, and persists upon restarting Maya.
You can also control this option by setting the environment variable MAYA_RENDER_SETUP_USE_UNTITLED_COLLECTIONS. A value of 0 disables the option, and a value of 1 enables it. The environment variable takes precedence over the preference. Once the environment variable is set, the Options > Enable untitled collections when new objects are created menu option is greyed out and follows the value of the environment variable.