Expand static attributes explanation

Author: jason-fox

docs/iot-agent-json.md

@@ -497,7 +497,16 @@ curl -iX POST \
 
 In the request we are associating the device `motion001` with the URN `urn:ngsi-ld:Motion:001` and mapping the device
 reading `c` with the context attribute `count` (which is defined as an `Integer`) A `refStore` is defined as a
-`static_attribute`, placing the device within **Store** `urn:ngsi-ld:Store:001`
+`static_attribute`, placing the device within **Store** `urn:ngsi-ld:Store:001`.
+
+> Static attributes are useful as additional data on an entity to enable querying using the `q` parameter. For example the Smart Data Models [Device](https://github.com/smart-data-models/dataModel.Device/blob/master/Device/doc/spec.md) model defines attributes such as `category` or `controlledProperty` which enable queries to be made like:
+>
+>  -  _Which **Actuators** currently have a low `batteryLevel`?_
+>  -  _Which **Devices** measuring `fillingLevel` were installed before January 2020?_
+>
+>  Obviously static data can be extended as necessary and can also include additional data such as a unique `name` or `serialNumber` for each device should the entity ID be too inflexible for queries.
+>
+> Additional devices with a fixed `location` static attribute can also be queried using the Geofencing parameters.
 
 You can simulate a dummy IoT device measurement coming from the **Motion Sensor** device `motion001`, by making the
 following request

docs/iot-agent.md

@@ -483,7 +483,16 @@ curl -iX POST \
 
 In the request we are associating the device `motion001` with the URN `urn:ngsi-ld:Motion:001` and mapping the device
 reading `c` with the context attribute `count` (which is defined as an `Integer`) A `refStore` is defined as a
-`static_attribute`, placing the device within **Store** `urn:ngsi-ld:Store:001`
+`static_attribute`, placing the device within **Store** `urn:ngsi-ld:Store:001`.
+
+> Static attributes are useful as additional data on an entity to enable querying using the `q` parameter. For example the Smart Data Models [Device](https://github.com/smart-data-models/dataModel.Device/blob/master/Device/doc/spec.md) model defines attributes such as `category` or `controlledProperty` which enable queries to be made like:
+>
+>  -  _Which **Actuators** currently have a low `batteryLevel`?_
+>  -  _Which **Devices** measuring `fillingLevel` were installed before January 2020?_
+>
+>  Obviously static data can be extended as necessary and can also include additional data such as a unique `name` or `serialNumber` for each device should the entity ID be too inflexible for queries.
+>
+> Additionally devices with a fixed `location` static attribute can also be queried using the Geofencing parameters.
 
 You can simulate a dummy IoT device measurement coming from the **Motion Sensor** device `motion001`, by making the
 following request