What is this syntax: $[] - it's referenced in docs but not explained in the Variables section

There are several references using $[...] syntax in the serverless-plugin-aws-alerts docs: Serverless Framework: Plugins

I understand about ${…} variables from the relevant docs: Serverless Framework Variables

But I can’t find anything to describe what is happening in the below code snippet (taken from the aws-alerts plugin docs linked above)

        nameTemplate: $[functionName]-Duration-IMPORTANT-Alarm # Optionally - naming template for the alarms, overwrites globally defined one
        prefixTemplate: $[stackName] # Optionally - override the alarm name prefix, overwrites globally defined one

It won’t let me post the reply due to some spam filter. The full explanation is here: What is this syntax: $[] - it’s referenced in Serverless Framework docs but not explained in the Variables section - Stack Overflow