Default Job Properties allow you to set the default job properties for a setup. All jobs manually added to this setup will inherit the default job properties. Jobs sent from a client application will use the job properties set in the client application.
This is useful because it allows you to create multiple specialized setups for each output device. One setup might have the default settings for a proof copy, for instance, while another might be preset for final output. Using multiple setups in this way prevents you from having to change the job properties for each stage of each job.
You can also set the default job properties by clicking Save as Default in the Job Properties dialog.
See Setting Job Properties for more information.