SD Item category

Item category in SD is used as an indicator that defines the characteristics of a document item.

For example, the item category controls the type and scope of: Pricing, Billing, Delivery,Inventory posting, Transfer of requirements,Items in stock, and value and text items are item categories.

As a customizing part you can define and assign item categories.
As an assignment process follow the steps as below:
Transaction SPRO- IMG->Sales and Distribution->Sales->Sales Documents->Sales Document Item->Assign Item categories.