Custom sliders or faders are normally used to collect sliders which don’t fit into the other predefined panels.
This is function control which is not direct lamp, beam, movement, gobo or color which can be placed in this panel.
All function control can of course be done by using the right side sliders link but there all sliders are present. Normally only relevant sliders are placed in custom sliders so it makes the fixture easier.
Panel Description - Sliders
See eventually also Fixture Creator - Custom sliders configuration