update ARM template user-defined functions article

Author: mumian

articles/azure-resource-manager/templates/user-defined-functions.md

@@ -3,14 +3,12 @@ title: User-defined functions in templates
 description: Describes how to define and use user-defined functions in an Azure Resource Manager template (ARM template).
 ms.topic: conceptual
 ms.custom: devx-track-arm-template
-ms.date: 04/28/2025
+ms.date: 07/17/2025
 ---
 
 # User-defined functions in ARM template
 
-Within your template, you can create your own functions. These functions are available for use in your template. User-defined functions are separate from the [standard template functions](template-functions.md) that are automatically available within your template. Create your own functions when you have complicated expressions that are used repeatedly in your template.
-
-This article describes how to add user-defined functions in your Azure Resource Manager template (ARM template).
+User-defined functions help simplify and reuse complex expressions in your ARM templates. By encapsulating logic in custom functions, you can improve template readability, maintainability, and scalability—especially when the same logic appears in multiple places. These functions are distinct from the [built-in template functions](template-functions.md) that are always available. To define user-defined functions in Bicep, see [User-defined functions in Bicep](../bicep/user-defined-functions.md).
 
 ## Define the function
 
@@ -65,7 +63,7 @@ The following example shows a template that includes a user-defined function to
         ],
         "output": {
           "type": "string",
-          "value": "[concat(toLower(parameters('namePrefix')), uniqueString(resourceGroup().id))]"
+          "value": "[format('{0}{1}', toLower(parameters('namePrefix')), uniqueString(resourceGroup().id))]"
         }
       }
     }