Explanation
A blanket purchase order is the best suited type of purchasing document for this purpose. A blanket purchase order is a long-term agreement with a vendor for several minor repairs that may occur over a period of time.
You can specify an overall limit value for the blanket purchase order and enter the account assignment information without specifying any material or quantity details. References: SAP S/4HANA Sourcing and Procurement | SAP Learning, Course S4520, Unit 2, Lesson 1.

Explanation
A business partner is a person or organization that has a business relationship with your company. When creating a new business partner, you can choose from the following categories:
Organization, which represents a legal entity such as a company, corporation, or institution.
Person, which represents an individual such as an employee, customer, or contact person.
Customer, which represents a business partner who purchases products or services from your company.
References: SAP S/4HANA Sourcing and Procurement | SAP Learning, Course S4500, Unit 1, Lesson
1.

Explanation
To issue delivery reminders for a purchase order, you must meet the following prerequisites:
Output the purchase order, which means that you send the purchase order to the vendor by print, fax, e-mail, or EDI (electronic data interchange).
Maintain reminder periods in the purchase order item, which specify how many days before or after the delivery date you want to send the reminder and how many reminders you want to send.
Set message determination in Customizing and maintain message records, which define the output type, medium, time, and partner for the delivery reminder. References: SAP S/4HANA Sourcing and Procurement | SAP Learning, Course S4520, Unit 2, Lesson 1.