As per the qeuery you backup about the media selection for the backup Job
When ever you run backup 1st it check if the media to which it can write is available in the media set where the backup is targeted . If there is no media to append or to overwrite it would look for scract media or imorted media as per the option you have set under Tools | option | Media mangment
Overwrite scratch media before overwriting recyclable media contained in the targeted media set
Overwrite recyclable media contained in the targeted media set before overwriting scratch media
Please refer the following document http://support.veritas.com/docs/192265 and this will answer you questions Much clearly ...