Customer number on various documents

Please could someone assist.

We have a client (JDE client), that has a request from their customers to print the customer's own number on their documents, e.g. invoices, statements etc.

Will try and explain.  The client has created the customer in address book and AR record.  This number currently prints on all documents sent to the customer.  The customer however wants their own customer number (from their system) to print on all the documents sent to them.

I am looking for a field we can populate to be able to give them this number.  Is there some sort of cross-reference for customer number, NOT Item Cross Reference.