The field for the device is specifically reserved for indicating the path status of the device.
--> When the device has more than one path to the target( storage array), the path status is "on".
--> When all the paths to the target are down ( either off / dead ) the device status is "dead".
--> If there is only one path to the target then the status is "degraded".
--> When a device is unmapped from the storage array while ESX was using the device the status of the device is "not connected".
--> If ESX fails to recognize the state of the device ( if all above mentioned scenarios are not applicable) then the device status is "unknown".