Every device type is associated with a generic device group or generic. A generic is an MVS-defined grouping of devices with similar characteristics, which determines how devices are used in an esoteric device group. Each generic has a preference number which governs the order of allocation of devices to jobs within an esoteric. For example, if the esoteric contains the device types 3990, 3880 and 3850, and the generics associated with the device types have preference numbers 280, 290, and 300, respectively, then the preference numbers are used by the z/OS allocation component to assign the devices in that order of preference.
Faster devices have lower preference numbers. You can change the preference number of a generic under an EDT (user-defined generic), but not under an OS configuration (system-defined generic).