When should we use a parameter when to use variable in ARM templates

In the Azure template json file:

parameters:Values that are provided when deployment is executed to customize resource deployment.

variables:Values that are used as JSON fragments in the template to simplify template language expressions.

More information please refer to this official document:Understand the structure and syntax of Azure Resource Manager templates.

I am more curious of the justification of using variables.

Based on my experience, if you only use the variable once, you don't need use variables. But if you want to use the variable multiple times, you had better use variables. Using variable can simplify your template to avoid duplication of content.

For example, if you don't use supportLogStorageAccountName more than once, you can just do:

"name": "[toLower(concat('sf', uniqueString(resourceGroup().id),'2'))]"

However if you use provide variable supportLogStorageAccountName several\many times, you can use variable to avoid duplication.

ARM templates are usually used to create a set of close to identical environments. The parameters are what differs them. This is commonly used for environment type (prod, dev, test) and performance/cost related parameters. Variables are used to create unique names for services based on or calculated from the parameters.

An example of this would the name of a storage account. This is usually done by concatenating a common name like _storage and an environment name parameter like “test” and store it in a variable. When you create another environment all you must do is change the environment type parameter.