Field help (BPCORDSIH) 

This information cannot be modified and is initialized by the bill-to customer code in the case of a direct invoice or credit memo or by the sold-to customer for the document to be invoiced in the case of the invoicing of orders, shipments, service contracts, etc.

It is possible to search a customer or several customer grouped under the same criteria by selecting "Quick customer search". A list of matching items is generated on tabulating to the next field.
For a more advanced search, all fields present in the block can be entered. The list of matches is narrowed down with each tabulation.