Explanation
You can configure the following assignments:
The allowed account assignment categories for each item category, which determine whether you can assign a purchase requisition or purchasing document item to an account assignment object such as cost center, order, project, or asset.
The allowed item categories for each document type, which determine whether you can use a specific item category in a purchase requisition or purchasing document with a certain document type.
References: SAP S/4HANA Sourcing and Procurement | SAP Learning, Course S4520, Unit 3, Lesson
2.