Once you patch a fixture in an FLX S desk using Setup>Add Fixture it is assigned to a Multi Function fader that you have selected as the Fixture Number.
If you want to remove the fixture you will be offered two choices, Unpatch or Delete.
If you Unpatch the fixture, the DMX address is removed and there will be no fixture information shown in the Fixture Schedule window or on the Multi Function Fader in the Output Window, but the Multi Function fader cannot have a new fixture allocated to it by Patching as the Multi Function Fader is reserved.
The Multi Function Fader will not appear in the Unpatched Fixtures on the lower tab bar of the Fixture Schedule window. If you have a lot of different fixture types, you may need to slide the tabs to the left to see the Unpatched Fixtures tab as it is the last tab on the right.
You can select the DMX address of the unpatched fixture in the patch window and assign a new DMX address or you can Delete it.