A power modifier is a metatrait, a collection of Enhancements or Limitations along with a specified Source.
GURPS Powers and GURPS Psionic Powers focus on this concept more, expanding from Basic Set.
It is possible to have TWO sources combined to price out a merged power modifiers. This is outlined in Stacking Power Modifiers on page 28 of Powers:
- No advantage can have multiple power modifiers.
- (Exception: A power can be both Elemental and Super.)
This would stack their modifiers to -20% and is mentioned at:
- 124 for Cold/Ice
- 125 for Darkness
- 126 for Earth and Electricity
- 129 for Heat/Fire
- 134 for Sound/Vibration
- 136 for Water