To set a PO Prefix/ Start Number for your PO's go to Admin > PO Settings
Enter the Prefix you would like to set, you do not have to set a prefix if you would not like one.
Enter the start number in the box: 'Series of P.O. numbers start with:'
Enter the Minimum # of digits in a P.O. number.
If you wanted your PO's to have the prefix i.e. SD at the beginning with a start number of 1000 you would have it setup as the following:
*Note, you can change the PO number but you can only go up to a higher number than what you have it set as currently. For example, if you had your PO number already starting at 500 you can change it to anything past 500 but you cannot change it to a number lower than 500. If you make changes to the PO number this will only affect new PO's going forward.