The New Oracle Payables 11i provides excellent options for sequential voucher numbers assignment to each invoice and payment in order to ensure that each document has a unique number. I'll try and explain this by taking an example. Suppose you get two invoices with identical invoice numbers from two different suppliers. With Sequential numbering a user can assign each of these invoices a unique number. This enables the user to later on locate the invoice based on such unique number.

According to Oracle Implementation Consultants I know, one of the most important benefits of using sequential voucher numbers is that you can be sure that no document has been lost or unposted. Each of the uniquely assigned voucher number will have an audit record. This is true even if a user accidentally deletes a voucher. Any journal entry can be traced back to the originating document with the help of sequential numbering.

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Basic Steps in Setting up Oracle PayablesDefining Journal Entry Categories
Concept of Dynamic Insertion
See also Number Ranges in SAP