This morning we are testing our approach to custom numbering for purchase orders, sales orders (quotes), and invoices.
Custom numbering has been a common user request since we launched last September. The approach we've adopted will allow you to identify records using your own numbering scheme and use that number instead of the OfficeBooks assigned number on PDF reports (like invoices or POs).
At first, this "Reference number" field will be entirely free form. In a future release, we expect to offer some form of automation to allow you to "set and forget" your record numbers and have them follow an incrementation pattern you define.