The Performance % Complete field in P6 is utilized for Earned Value (EV) calculation and provides the percentage of the planned work that is currently completed. It serves as the initial field to assess the progress of an activity or project.

In Primavera P6, the Performance Percent Complete calculation technique is defined at the WBS level. 
Define which technique to use by going to Project, WBS, then selecting the Earned Value tab. This article do not explain how summary (rolled-up) performance % complete is calculated. It is discussed at this article:Why Performance % Complete doesn't roll up in Primavera P6?

Performance Percent Complete is used to calculate Earned Value based on the following:

Earned Value Cost = Budget At Completion * Performance percent complete

Earned Value Labor Units = BL Budgeted Labor Units * Performance percent complete

There are 5 different techniques for computing Performance Percent Complete in Primavera P6.

Activity % Complete:   Performance % complete is equal to the Activity % complete. This is the default and most used option.  This option means simply,
Performance Percent Complete = Activity Percent Complete = Whatever you selected as the activity % complete type (Duration % Complete, Units % Complete, Physical % Complete)  

Use WBS Milestones:  Calculate performance percent complete by defining milestones at the WBS level and assigning a level of significance or weight to each of them.  As progress occurs and you mark each milestone complete, the WBS element's Performance Percent Complete is calculated based on the weight of the milestone.

0/100 % Complete:   Performance Percent Complete is 0 until the activity is complete.  Once the activity is complete, Performance Percent Complete is 100.

50/50 % Complete:  Once an activity is started, Performance Percent Complete is 50.  Performance Percent Complete remains at 50% until the activity finishes, at which time the percent is 100.

Custom % Complete:  Manually enter a Performance Percent Complete at any WBS level. This percentage applies to all activities within the WBS level once an Actual Start is applied to the activity.   After the activity is completed, the activity's Performance Percent Complete is 100 percent.