What and Where comes the actual usage of LARAVEL_START const defined in index.php of Laravel 5.7?

When I'm looking into the Laravel Request Cycle, I encountered this line of code in the file index.php

define('LARAVEL_START', microtime(true)); 

I didn't find any specification about this in Laravel documentation, may be it's not Laravel specific, who knows. Also I can't find any usage of this either in the documentation or any other place, please help me to understand what this line of code actually for?

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