Admin can set discount rules based on Category, individually as well as on Category Combination.

To make this discount rules select Discount Rules tab.
Now choose Category Rules
Now you can choose Add Single Category Rule ( to make discount rule on a particular category ) or Add Combi Category Rule ( to make discount rule on a group of categories ).

Fill Rule, Restrictions and Validity tabs accordingly.

After saving the rule it can be seen in the given table below.

Now you have to give permission to your rule to give discount on the Category you have chosen.
Click on the Setting Tab.
Check Category Rule or/and Category Combination Rule as shown below.

After checking these rules, Choose among First Match Rule, Best Match Rule or All Match Rule and click on save rule button below.

The Discount can be seen on the shop page as well as on product page.

The discount can be shown in the cart if the products are added to the cart as in the given picture below.

Now whenever you want to disable this discount rule, you don’t have to delete it, just go to Setting Tab and uncheck this rule.