Typically, the discount rule for inventory models is based on the resource type. So if you add an inventory model to a Quote and the resource type of the line is set to Discountable = Yes, then the inventory model will be discounted. However, if you do not want to apply discounts to certain inventory models, you can set the item to not be discountable. This setting is located within the Pricing tab of the inventory model, in the bottom left corner.
The default value of the inventory model's "Discountable" setting is based on the inventory class-resource type that is listed first. In a default system, this would be the Rental resource type.
When "Discountable" is set to Yes for that resource type, you will be able to override the inventory model's discountable setting. When "Discountable" is set to No for that resource type, you will not be able to override the inventory model's discountable setting.
You can bulk update the inventory model's discountable field within the Inventory Worksheet, located on the Inventory menu. Just make sure that "View Options" is set to a pricing model and you'll see the Discountable column. Click here for more information on Inventory Worksheet.