When you start your trial with Zahara you are offered a purchase order start number. This can only be changed when you purge all of your orders. You are then presented with the chance to restart the numbering.
Purge & Restart
Head to Admin > Settings. Expand the Maintenance section
Ultimately the purchase order number is simply a reference used to identify the order on an invoice. Our numbers are globally sequential for the business unit. You can prefix them to identify different business units or divisions. Read the article on prefixes.