wrong account for customer in accounting document

A sales order has been created and basing on the order invoice has been created, In the subsequent accounting document the total value of the document is being posted to one specfc account. This is customized sales document. The document flow is SLAES ORDER, INVOICE, ACCOUNTING DOCUMENT. THe item category is manually entered for each level.

Please also note that the item category is entered manually in teh sales order. In all the standard sales orders, the total value of the order is posting to the respective customer accounts. But in the case this special document type the total value of the order is posting to 1 specfic GL account instead of to the customer account.

I would like to know how the system might pick up the customer number for the standard sales orders and the specfied GL account for the specfied document type.

How does the system assign the accounts on the accounting documents.