Here is the description:
Assimilation: The character with this proficiency is able to study a different culture well enough to pretend to be a member of it. Assimilation allows the character to pick up cultural mannerisms (common rituals, expressions of speech, taboos, etc.). It is distinct from Acting but helpful to that proficiency. A character who has both Acting and Assimilation proficiency receives a +1 bonus to checks with either proficiency when portraying a member of another culture. (This is not cumulative with the Acting/Disguise bonus; if a character has all three proficiencies, she does not receive a +2 bonus.)
Assimilation uses an INT check without modifier, and takes 2 slots for non-rogues, or one slot for rogues.