Explanation
A document type in purchasing is a classification of purchasing documents (such as purchase orders, contracts, requests for quotation, etc.) that defines some basic characteristics and controls for each document. Some of the aspects that you can control with a document type are:
Reference to purchase requisitions, which means that you can specify whether a purchasing document can be created with reference to a purchase requisition or not.
Number range, which means that you can define the number interval and the number assignment (internal or external) for each purchasing document.
Item number interval, which means that you can define the increment and the length of the item numbers for each purchasing document. References: [SAP Help Portal], [SAP Help Portal], [SAP Help Portal]