Sometimes project costs need to include line-item charges and/or discounts. If you want your cost model to include line-item charges and discounts, when you create your cost model click Add in the "Additional Charges and Discounts" section.
To specify additional charges or discounts
When modifying a cost model, or a cost model rule, keep in mind that any changes will impact cost and scoping information for all existing projects to which the cost model currently is applied.