Dear Sir,
Heavy cut compound will be used on extremely contaminated surfaces only, it should never be used on fresh paints or new cars.
It is used on repainted surfaces to make the paint even, or on highly rusty or contaminated paints.
Also, it works in combination with step 2 (Mild) and step 3(Fine)
It means, after using hard compound, you must have to treat the surface with step 2 mild compound and then step 3 for final glossy results.
For low contamination or new paints, you can directly use Step 3 compounds or step 2 if contamination is more.
Hope it clears!