Multiple Categories Versus Sub-Categories

If I add a second category to an item will it simply appear under both root level categories? What then would be the significance of the main category radio button selection?

For example, if I have an item “Mona Lisa” (it's an upscale site) with Art as its main category and an additional categories of Paintings and Antiques added to it, would this simply list “Mona Lisa” under all three categories, as Art>Mona Lisa, Antiques>Mona Lisa, Paintings>Mona Lisa?

Would it cause “Mona Lisa” to be listed in an Art sub-category of both Antiques and Paintings, as in Antiques>Art>Mona Lisa and Paintings>Art>Mona Lisa? This second seems strange and if this kind of nesting takes place it would seem that categories need to be chosen carefully. In this case all Paintings should have Art as main category (would this be the only way to create a proper sub-category?).

Would it Cause “Mona Lisa” to be listed in two sub-categories of Art, Art>Paintings>Mona Lisa and Art>Antiques>Mona Lisa?

How does this work in general? Thanks for any help.