Merge pull request #206230 from dominicbetts/central-reserved

IoT Central: device payload claifications

Author: PRMerger13

articles/iot-central/core/concepts-telemetry-properties-commands.md

@@ -22,6 +22,9 @@ A device template in Azure IoT Central is a blueprint that defines the:
 
 This article describes the JSON payloads that devices send and receive for telemetry, properties, and commands defined in a device template.
 
+> [!IMPORTANT]
+> IoT Central expects to receive UTF-8 encoded JSON data.
+
 The article doesn't describe every possible type of telemetry, property, and command payload, but the examples illustrate all the key types.
 
 Each example shows a snippet from the device model that defines the type and example JSON payloads to illustrate how the device should interact with the IoT Central application.
@@ -47,6 +50,18 @@ IoT Central lets you view the raw data that a device sends to an application. Th
 
 ## Telemetry
 
+To learn more about the DTDL telemetry naming rules, see [DTDL > Telemetry](https://github.com/Azure/opendigitaltwins-dtdl/blob/master/DTDL/v2/dtdlv2.md#telemetry). You can't start a telemetry name using the `_` character.
+
+Don't create telemetry types with the following names. IoT Central uses these reserved names internally. If you try to use these names, IoT Central will ignore your data:
+
+* `EventEnqueuedUtcTime`
+* `EventProcessedUtcTime`
+* `PartitionId`
+* `EventHub`
+* `User`
+* `$metadata`
+* `$version`
+
 ### Telemetry in components
 
 If the telemetry is defined in a component, add a custom message property called `$.sub` with the name of the component as defined in the device model. To learn more, see [Tutorial: Create and connect a client application to your Azure IoT Central application](tutorial-connect-device.md).
@@ -436,6 +451,8 @@ A device client should send the state as JSON that looks like the following exam
 
 ## Properties
 
+To learn more about the DTDL property naming rules, see [DTDL > Property](https://github.com/Azure/opendigitaltwins-dtdl/blob/master/DTDL/v2/dtdlv2.md#property). You can't start a property name using the `_` character.
+
 > [!NOTE]
 > The payload formats for properties applies to applications created on or after 07/14/2020.
 
@@ -869,6 +886,8 @@ The device should send the following JSON payload to IoT Central after it proces
 
 ## Commands
 
+To learn more about the DTDL command naming rules, see [DTDL > Command](https://github.com/Azure/opendigitaltwins-dtdl/blob/master/DTDL/v2/dtdlv2.md#command). You can't start a command name using the `_` character.
+
 If the command is defined in a component, the name of the command the device receives includes the component name. For example, if the command is called `getMaxMinReport` and the component is called `thermostat2`, the device receives a request to execute a command called `thermostat2*getMaxMinReport`.
 
 The following snippet from a device model shows the definition of a command that has no parameters and that doesn't expect the device to return anything: