User wants to manually change % Complete.

Percent Complete:

If the selected activity's percent complete type is set to Duration, the selected activity's duration percent complete is calculated from the Original or Planned and remaining duration.

If the selected activity's percent complete type is set to Physical, this field displays the selected activity's physical percent complete, which is manually entered.

If the selected activity's percent complete type is set to Units, this field displays the selected activity's unit percent complete, as calculated from the actual and remaining units.