You can enter external document numbers for customers and vendors in all sales and purchase orders, invoices, and credit memos. You can use these numbers to search for posted sales and purchase lines later.
Note
Specify the document number that your customer or vendor has specified. Business Central doesn't check if these external document numbers are unique or duplicates.
The following procedure explains how to specify an external document number on a sales order, but similar steps apply to other sales and purchase documents.
To enter external document numbers in a sales order
Select Search (Alt+Q) 
in the upper-right corner, enter Sales Orders, and then choose the related link.
Select the relevant sales order, and then choose the Edit action.
On the General FastTab, in the External Document No. field, enter the invoice number.
On purchase documents, the relevant field is the Vendor Invoice No. field.
Fill in any other required fields.
Related information
External Document Numbers on Purchase Documents
External Document Numbers on Sales Documents
Sell Products
Purchasing
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