You enter a two character key in the Posting key field in order to specify how a line item is posted. The following figure illustrates the posting key function:

The most important posting keys for customer line items are:
Your system administrator can define further posting keys should you need them.
The document type entered in the header and the posting key are related. The document type specifies which account types you can post to. The posting key specifies that you can post to a particular type of account. A posting key can only be entered for an account type which is permitted for that document type.