Before test job,we need to add a "device_type" .jinja2 file to directory /etc/lava-server/dispatcher-config/device-types/,and add a "device" .jinja2 file to /etc/lava-server/dispatcher-config/devices/.
The "device_type" .jinja2 file defines device type(for example different hardware)  ,and "device" .jinja2 file(at: /etc/lava-server/dispatcher-config/devices/) defines a concrete device.
When the  dispatcher submits job, a job should be submited to a concrete device.But In job defintion file,the description of the related device is only the "device_type" but no "device", why is not "device"? 
for example:
# Your first LAVA JOB definition for arm
device_type: arm
job_name: arm pipeline, first job
  ## why not as:
# Your first LAVA JOB definition for arm
device: arm01
job_name: arm pipeline, first job