In Sage 50, every time you update an invoice, make or receive and payment, make a journal entry or do anything which impacts the nominal ledger, Sage creates new transaction records, and numbers them sequentially starting from 1. This is called the Audit Trail and can be viewed by clicking Transactions on the left menu ...
Historically, these transaction numbers were not static. If you decided to clear your audit trail after the year-end, Sage 50 would delete any transactions which had been completely finished with (see my post on Spring Cleaning your Accounts Software for more information). The system would then create new opening balances and rebuild the audit trail starting with 1, and all the transactions would be renumbered.
The problem is that many companies, when doing their bookkeeping, would use this transaction number as a unique ID to identify invoices or journals which meant that tracing historical data after clearing the audit trail could prove difficult.
Sage Support used to get calls about these numbers changing quite regularly, and they changed the system so that once a transaction is entered and allocated a number, it will always keep this number.
This means that if you have done a Clear Audit Trail you will now have missing transaction numbers which represent those transactions from previous years which have met the criteria to be cleared down. This change, however, was not well publicised, so now Sage users query why there are missing transaction numbers.
If anything I've written here resonates with you, call me on 01604 420057 and let's see how we can help you.