If a control property is set to invisible, is it automatically disabled? If it was just invisible, could the user still technically tab to it if tab stop was enabled and press the space bar to activate it (i.e., a button)?
Users could not interact with an invisible control - there's no way to edit it or tab to it. But it is not automatically disabled and you could still use the invisible control via your form code or by way of references to it.
This makes for a lot of possible ways to control access to controls - but I assume in general you disable controls that aren't needed in context, lock controls you don't want the user to edit but don't mind if they see, and hide controls that are irrelevant to the user but may assist in the "behind the scenes" functioning of the form.