Discounts can be applied to groups of retail products at POS, by using display categories. 

Go to System>Setup>Discounts Setup>Create New

Select POS/Online Shop

Add Details

Select Display Categories

First we must enable restrictions and then select one or more display categories. This discount will then apply only to retail products or items assigned to these display categories. See Display Categories for more information.

Example Retail Products

The Beanie and Cap retail products below are listed under Hats (the display category selected above in discount setup. See Display Categories for more information.

Beanie

Cap

Apply at POS

Select product i.e. Beanie.

Select ‘Discount Surcharge’ Button.

Apply Discount just created.

See applied and payment ready to be made.

If try to apply to product that is not under Hats category; then will not apply.