Basic document control principals can help make your business run more efficient. Order Time will automatically create a sequence for the document numbers used which will establish a unique identifier for each transaction type.
In order for this sequence to be followed the characters need to be numeric. The sequence of the numbers will not only help you identify the order in which the documents were created, but also help you identify if any documents were deleted.
Keeping the sequence of the documents is a very important internal control to ensure operational effectiveness and reliability in your documents. It will help in revealing any discrepancies during an audit process.
If you require an alphanumeric number consider Creating a Custom Fields which can be printed on custom forms and reports.