This dialog is accessed from numerous locations within the Schema Configuration interface allowing you to select a new class to add:

The field at the top of the dialog lets you filter the display by entering the filter criteria into the field (i.e. - Pump, Equipment, etc) then clicking the Search icon to apply the filter.

To clear the filter click the Clear Search icon to return to the default display.

Select a class to include and click the Select Class button to add it to the Schema Configuration utility.