WO / PO / Invoice Numbering
You can configure automatic numbering for your Work Orders, Purchase Orders, and Invoices, set the starting point and apply an optional prefix for the numbering system.
Automatic numbering is a "per user" setting which is enabled by default. If you have the network version with multiple client installations, this setting is available to adjust on each client.
Navigate to Setup | User Preferences.
Choose from Work Orders, PO or Invoices on the left, then check the corresponding box under "Auto Number" and include an optional Prefix if desired.
The starting point is a company-wide setting which only needs to be established once. For this, navigate to Setup | Settings. Click the Program Settings tab.