There are two ways in which jobs may be assigned to a Printer:
Auto - automatically assigns the print job to a Printer that: 1. Uses that job's material; and, 2. Is marked as "ready to print." This is the recommended/default setting, as it assures print jobs the quickest path to getting started on a Printer.
Manual - assigns the print job to specific Printer chosen by the operator. This is recommended if each of your Printers has a specific type of print job it runs.
You can select either option in the job-submission screens, to control this on a per-job basis. In addition, you can set this on a system-wide basis by going to Settings > System Controls and tapping either the Auto or Manual button.