This can be applied to any advantage that alters how others perceive you, such as Appearance, Chameleon, Elastic Skin, Invisibility, Shapeshifting (with the Cosmetic limitation), Silence, or Voice. It has only effects living beings so machines and undead will see or hear through the illusion.
GURPS Powers pg 111
GURPS Power-Ups 8 pg 13-14
It has no cap on the value, so something that gives a huge bonus to the will roll could be worth a LOT, like Will+10 would be -80%