::Hi Canadian Guy,
There are already fields for these ID’s.
In Purchasing the Document Number is the Vendor’s Invoice Number and the Voucher Number is YOUR key field for that purchasing document – you should rarely be changing the Voucher Number unless you were specifically asked to.
In Sales, the Invoice Number is YOUR invoice number to the customer and is 100% unique. The PO Number field in Sales (whether Receivables or Sales) is good for Customers PO Number, which is generally what you are getting from a Customer)
If you are trying to use a separate source-systems invoice information, if it is done right, you can use those source-system numbers as your sales-invoice numbers already – you can use it in the integration to replace the default invoice numbers in GP.
Otherwise, let me advise against a developer just for User Defined (UDF) Fields in GP – the best product there is for this is eOne’s Extender and you can create tons of custom fields and report on those as well.