The Power Point Reserve:
Psionic characters fuel their abilities through a pool, or reserve, of power points. Your power point reserve is equal to your base power points gained from your class, bonus power points from a high key ability score (see Abilities and Manifesters), and any additional bonus power points from sources such as your character race and feat selections.
Multiclass Psionic Characters:
If you have levels in more than one psionic class, you combine your power points from each class to make up your reserve. You can use these power points to manifest powers from any psionic class you have. While you maintain a single reserve of power points from your class, race, and feat selections, you are still limited by the manifester level you have achieved with each power you know.
Abilities And Manifesters:
The ability that your powers depend on—your key ability score as a manifester—is related to what psionic class (or classes) you have levels in. The modifier for this ability is referred to as your key ability modifier. If your character’s key ability score is 9 or lower, you can’t manifest powers from that psionic class. Your key ability score grants you additional power points equal to your key ability modifier × your manifester level ×½. The following table shows these calculations for class levels 1st through 20th and key ability scores from 10 to 41.