The 9 box Talent Matrix or Succession Pool will plot your employees out based upon their overall performance score, along with a potential score assigned by the manager. Mouse over the initials to view the employee's scorecard and use the filter options to modify view.
If you are not seeing any initials in the boxes after an evaluation has been completed, make sure the employee's Potential Rating has been added. (See Potential Rating)
The only way to move an employee within the 9 box, is to change their performance ratings within the evaluation, or adjust their potential rating.