I remember that there being 3 levels of summoning...
#1 - Summon after some damage is done. This is the most common variety as I remember it. This was a % of the mobs total HP (2-3%), I know this for sure because I would sometimes go and try killing raid mobs and if I felt like I was not going to win the fight I could somtimes stop attacking long enough for them to regen back up to 97% and then I could escape because they would stop summoning. Granted this was with Luclin raid mobs (Ssra mobs primarily), but I think it is the general rule.
#2 - Summon on aggro. This was a special case..some mobs would summon with no damage done. As soon as they had aggro they would start summoning. I know I remember this being in, I just cannot recall any mobs off the top of my head that did it.
#3 - Summon with Spells. Specifically Call of the Zero, but I think all of these were Velious Raid mobs
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