As informações de que você precisa estão em Regras do Monstro Universal .
Summon (Sp) A creature with the summon ability can summon other specific creatures of its kind much as though casting a summon monster spell, but it usually has only a limited chance of success (as specified in the creature's entry). Roll d%: On a failure, no creature answers the summons. Summoned creatures automatically return whence they came after 1 hour. A creature summoned in this way cannot use any spells or spell-like abilities that require material components costing more than 1 gp unless those components are supplied, nor can it use its own summon ability for 1 hour. An appropriate spell level is given for each summoning ability for purposes of Will saves, caster level checks, and concentration checks. No experience points are awarded for defeating summoned monsters.
Então, é uma chance de sucesso de cada uso separado dessa habilidade. Você rola d% cada vez que uma criatura usa a habilidade para determinar se foi um sucesso ou não.