Individual billing types take several different forms.
The differences lie mainly in:
Which documents may be used as the basis for the billing document?
Which data is to be copied to the billing document from the reference document?
Should the order quantity or delivery quantity be invoiced?
According to which requirements is a sales document to be billed?
Should pricing be carried out again or should prices be copied from the sales order?
These control options are defined by your system administrator in SD Customizing via Billing ® Billing documents ® Maintain copying control for billing documents.