The TBD Planning Resources selection criteria can be used to indicate whether those users having the TBD Planning Resources designation are to be included in the report capacity calculations.  

You will also have the option to indicate whether the resulting output will include Plans or Budgets for the TBD Planning Resources.

The TBD Planning Resource designation is set on the Person Profile >> Roles tab.

This option will only appear if your installation is running with the Project Portfolio Plus or Purchasing license.