It is now possible to select devices in REST API calls based on the values entered in item-fields plugin.
For example if you have plugin with customer_id or vehicle_plate_number field defined you may select devices with specific field values this way: plugins.fields.customer_id=123 OR plugins.fields.vehicle_plate_number=ER89. Like this: GET /gw/devices/plugins.fields.customer_id=123/telemetry/position.
And you do not have to use 'Store' option enabled for this. It will work with current value in the plugin field defined for the device.
This will give you the possibility to define own set of data fields for the device that you may utilize in REST API calls to select devices and this data may not be visible in device messages or even in the flespi account where device is located.