A Document type is specified at the Header level during transaction entry and serves the following purposes:
It defines the Number range for documents.
It controls the type of accounts that can be posted to eg Assets, Vendor, Customer, Normal GL account.
Document type to be used for reversal of entries.
Whether it can be used only for Batch input sessions.
Document Type is created for differentiating business transactions. Eg Vendor Invoice, Credit Memo, Accrual Entries, Customer Invoice. It is a two digit character.