The idea of a document outside SAP, may be just a piece of paper where information is placed.
But a the Document Type in SAP represents a lot more, because it has many properties, the main ones are listed here:
It allows you to differentiate between the business transactions to be posted. The document type tells you what sort of a business transaction it is. This is useful, for example, when displaying line items for an account. The assignment of document types to business transaction variants is of central importance in document splitting (see Making Settings for Document Splitting ).
It allows you to control how postings are made to account types (vendor, customer, or G/L accounts). The document type determines the account types to which postings can be made with that document type.
It allows you to assign document numbers. A number range is assigned to every document type. The numbers for the documents you create are taken from this number range. The original documents from one number range should be stored together. In this way, the document type controls document storage.
In order to learn more about Document Types and their configuration, refer to our course in Udemy.