Explanation
Centrally agreed contracts are contracts that are created by a central purchasing organization and can be used by multiple purchasing organizations within a company. Some of the characteristics of centrally agreed contracts are:
You can use centrally agreed contracts in a procurement hub scenario, which means that you can leverage the central purchasing organization's negotiation power and expertise to procure materials or services for other purchasing organizations.
You cannot specify plants in centrally agreed contracts, which means that the contracts are not plant-specific and can be used by any plant that belongs to the purchasing organizations that are assigned to the central purchasing organization. References: [SAP Help Portal], [SAP Help Portal]

Explanation
When you post an invoice with invoice reduction, two accounting documents are created: one for the invoice posting and one for the credit memo posting. A message is also created that can be issued to the supplier as a notification of a credit memo posting. The invoice is not blocked for payment until the supplier confirms the credit memo receipt, and only one accounting document is not created, containing both the invoice and the credit memo postings.References: [SAP Help Portal - Invoice Reduction]

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.

Explanation
The account category reference is a grouping of material types that have a similar account determination. By assigning the same account category reference to different material types, you can control that they use the same valuation class for account determination. The valuation class is a key factor that determines which general ledger accounts are posted during inventory transactions. Therefore, if you want to set up the same account determination for raw materials (material type ROH) and operating supplies (material type HIBE), you can group them by assigning the same account category reference to these material types.References:
[SAP Help Portal], [SAP Help Portal]