SAP DOCTYPE means

The document type is used for the following functions in General Ledger Accounting (FI-GL), Accounts Receivable (FI-AR), and Accounts Payable (FI-AP) components:

• Differentiating the business transactions to be posted

All vendor invoices, for example, are posted using the same document type.

• Controlling document storage

This is the main function of the document type. You must specify a number range for every document type. The original documents from a number range are stored together.

• Posting invoices with the vendor net procedure

The document type determines whether or not the net procedure is used when posting a vendor invoice. With the vendor net procedure for posting, the system automatically subtracts cash discount from the corresponding offsetting entries when the document is posted.The document type is entered in the document header and applies to the whole document.
Document types are defined at client level. The standard system already contains document types which you can use or change.