Foster provides a list of eight different categories to which a “person suspended in air” might belong. Why are these (or a combination of these) the only eight things that suspended person could be?

From the text:

Meaning that when we see a person suspended in the air, even briefly, he is one or more of the following:

1) a superhero

2) a ski jumper

3) crazy (redundant if also number 2)

4) fictional

5) a circus act, departing a cannon

6) suspended on wires

7) an angel

8) heavily symbolic