Step-1 : General tab 

In this example, you will be knowing how to apply discount for specific category.  On general tab give the order number, Rule name, Rule description and then choose
Method = Quantity based by product/category and BOGO deals

Step-2: Condition tab 

On condition tab, choose
Apply to = Specific categories and choose the category to which you wants to apply the discount. For example, we choose "Apparels"
Check the box "Apply child categories", if you would like to include the sub-categories of Apparels.
Click Next.

Step-3: Discount tab 

Define the quantity range and give the discount value 10.
Min quantity - 1
Max quantity - 99
Adjustment type - Percentage discount
Value - 10 

Step-4: Frontend demo 

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