Many Improved Familiars (e.g. ratling, imp, faerie dragon) have spell like abilities (SLAs). In each's default monster listing, it lists a caster level (CL) for them, e.g. (from ratling)
Spell-Like Abilities (CL 6th; concentration +7)
Familiar Basics changes a number of things for creatures that become familiars, including
Hit Dice: For the purpose of effects related to number of Hit Dice, use the master's character level or the familiar's normal HD total, whichever is higher.
Does this increased HD get used as the CL for the familiar's SLAs? Or does the CL always stay whatever's listed in the familiar's default stat block?
Best Answer
No, Familiar HD and CL do not increase.
The 'effects' referenced in your quote are actually spells that would target it like Sleep. For the purpose of abilities, the rule
supersedes the other. It's HD are static and unlike Animal Companions, it does not gain more HD or anything of the like from its master's EWL.
I will clarify that these are RAI, and this is a highly debated issue that it seems like Paizo will errata at some point. See my related post answer for a more in-depth response and the quote from a Paizo Contributor that this is based on. A GM could easily decide either way with it, but I will leave you with the point that this Reddit post shows some of the dangers of allowing Familiars to use their master's HD for DC calculations.