Den information du behöver finns i Universal Monster Rules .
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.
Så det är en chans att lyckas med varje separat användning av denna förmåga. Du rullar d% varje gång en varelse använder förmågan att avgöra om det var en framgång eller inte.