Document 
A document is a record of a posting in Financial Accounting. There are two types of documents: original documents and processing documents.
Examples of original documents:
Receipts
Invoices
Checks
Bank statements
Examples of processing documents:
Accounting documents
Sample documents
Recurring entry documents
The accounting document represents the original document in the System. The other processing documents can be used to simplify document entry The document remains a connecting unit in the system until it is archived.
The accuracy of postings in the compact journal and the general ledger can only be checked on the basis of documents. For this reason, each posting must be based on a document.
Documents are the link in Accounting between the business transaction and the posting.
Note
Only complete documents can be posted. A document is complete when its debit and credit items balance to zero. You must enter the minimum account assignments designated by the system, for example, document date, posting date, document type, posting key, account number, and amount. Data must also be entered in all other fields that were defined as required fields when making system settings.
Structure
A document consists of a Document Header and at least two Line Items.
Integration
As well as the accounting document described above, there are also special documents such as sample documents and recurring entry documents. These are documents that simplify the document entry transaction. For more information, see Posting with Sample Documents and Recurring Entries.
You can transfer large volumes of document data from an external system into the SAP System. There are various methods for doing this. For more information, see Accounting Documents: Data Transfer Workbench.