Any of discount categories existing in the Control Center can be assigned to any of Reseller accounts. And this discount works similarly as for any customer accounts, i.e., to all purchases made for a reseller account.
To assign a discount category to a Reseller account, go to the Account Director - Reseller Manager - Resellers. Select a reseller account and then click the Financial Info tab.
If you want to create a special discount category for Resellers, you can do this in the Billing Director - Discount Manager - Discount Categoies.