How are defense groups defined?

[ollien]

I can't seem to figure this one out.

Section 5.5.10 states

The specific DEFENSES used during each MATCH are selected by a combination of the audience and the ALLIANCES. One (1) DEFENSE from each of the four (4) DEFENSE groups, A, B, C, and D are on the FIELD each Qualification and Playoff MATCH.

What are the defense groups? I can't find definitions for what defenses belong to what groups. Can someone help me out? Thanks!