Yes, identification of discount depends on account type.

1. Discount for Purchase
   1. Other Current Asset
   2. Fixed Asset
   3. Other Asset
   4. Other Current Liability
   5. Long Term Liability
   6. Equity
   7. Cost of Goods Sold
   8. Expense
   9. Other Expense
   10. Deferred Revenue
   11. Deferred Expense
2. Discount for Sale
   1. Income
   2. Other Income

You mean by restricting Discount for Purchase is that, you cannot select Discount for Purchase in Purchase Order or in other transaction? Or you do not want to select Discount for Sale in Sales Order? In either way, you can use saved search to filter the items that can be selected in item lines, then use this saved search in custom form > sublist fields > item filter.