When you create a new Job or an invoice in Onyx that includes a vehicle, Onyx will scan through all current estimates and if it finds one that matches the vehicle it will display a screen that shows you the estimates.  From the screen you can view the contents of the estimates or select them to have the details automatically entered into the job/invoice.

Similarly, if you create a new estimate, Onyx will show you a list of any existing, unconverted, estimates.  This is so that you don't accidentally create two estimates for the same job.