A THREAD FOR NEWCOMERS!

I started reading General Rules for Offline Heroes III Tournaments (since 06.10.2012) and came across a rule that specifies certain specific situations and techniques that I read about before:

5. During the game, it is forbidden to:
5.10. Raise undead from summoned elementals or from cloned monsters, or from demons raised (resurrected) using hellspawn in the hero's army, or from undead raised by an AI hero if your hero is defeated (killed/surrenders/flees).

I generally understand the gist of it, but I would like someone to explain this technique in more detail, as if explaining it to a beginner,: the nuances of this technique, in particular: whose elementals, etc., are used to raise additional undead, whether they are yours or your opponent's, etc., because I don't quite imagine it clearly since I have never encountered this in practice.