To create constraints between objects you must first add a constraint to your scene.
If you drag the constraint on top of an object in the Viewer window, a context menu appears asking you if you would like to constrain the object or set it as a source object in the constraint. If you drag the constraint into an empty part of the Viewer window, the constraint is added to the scene and you can selected in the Scene browser, but it is not connected to objects.
The Navigator window displays the Constraint settings, and the constraint is added to the Scene browser.
Scene browser A. Constraints folder
A new folder named “Constraints” appears in the Scene browser, with the new constraint added, and the Navigator window displays the Constraint settings.
See Snap button.
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