Update virtual-machine-scale-sets-health-extension.md

Author: ju-shim

articles/virtual-machine-scale-sets/virtual-machine-scale-sets-health-extension.md

@@ -26,6 +26,9 @@ This article assumes that you're familiar with:
 > [!CAUTION]
 > Application Health Extension expects to receive a consistent probe response at the configured port `tcp` or request path `http/https` in order to label a VM as *Healthy*. If no application is running on the VM, or you're unable to configure a probe response, your VM is going to show up as *Unhealthy*.
 
+> [!NOTE]
+> Only one source of health monitoring can be used for a Virtual Machine Scale Set, either an Application Health Extension or a Health Probe. If you have both options enabled, you will need to remove one before using orchestration services like Instance Repairs or Automatic OS Upgrades.
+
 ## When to use the Application Health extension
 
 The Application Health Extension is deployed inside a Virtual Machine Scale Set instance and reports on application health from inside the scale set instance. The extension probes on a local application endpoint and will update the health status based on TCP/HTTP(S) responses received from the application. This health status is used by Azure to initiate repairs on unhealthy instances and to determine if an instance is eligible for upgrade operations.