Add an irregular scatter of marks as a separate creative layer. Dust is distinct from the more continuous texture of Grain.
When to use it
Use it when a handled, imperfect surface suits the photograph.
How to control it
Amount controls strength; Color changes the tone of the marks; Randomize creates a different placement.
A starting approach
Randomize until important details stay clear, then adjust strength. The marks are generated effects, not a scan of actual film.
Combine with intention
Choose one main direction first. Compare with the effect reduced or off before adding another treatment. Check the exported photograph as well as the preview.