NSDocument is not all about UI. It's a model-controller class. is all about data with UI, therefore NSDocument without UI is NSFilePresenter. However I believe the idea was to NSDocument work without UI part - but that doesn't seems to be a thing anymore.
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This is a good intro, notice how the NSDocument controls the windows representing it, not the other way around.
The name is a little misleading for sure.
https://developer.apple.com/library/archive/documentation/DataManagement/Conceptual/DocBasedAppProgrammingGuideForOSX/KeyObjects/KeyObjects.html