Oh, that reference on page 12 is to normal summoning rules.
i.e. If I have a Tzeentch warband, and I have a sorcerer in my warband, I can summon a unit of Tzeentch daemons following the normal rules. i.e. summon a Daemon Prince on an 8+ or whatever the summoning rules are for that particular daemon on the war scroll.
Units you summon during a battle don't stay with your army, and would need to be summoned again in any future battle if you want to use them again.
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