This is vqmod extension. Generate the invoice number as soon as the order is placed.
Other mod will generate invoice number on oder, The Auto generated Invoice Number will always be the same as Order number so that you don't get confuse when you look at order number and invoice number.
Auto generate invoice number is suit for anyone want the invoice number generated on orders .
Example: Order number is 1234
Atuo generated Invoice number will be INV-1234 (depend on your settings, some use INV-2012-00, it will become INV-2012-001234)