Explanation
The account assignment category controls the following aspects of a purchase requisition or purchasing document item:
The type of account assignment object you must specify, such as cost center, order, project, or asset.
Whether you must enter a material in the purchase order or whether you can enter a short text and material group instead. References: SAP S/4HANA Sourcing and Procurement | SAP Learning, Course S4520, Unit 3, Lesson 2.