The composer.json file in https://github.com/symfony/skeleton/blob/6.4/composer.json contains the property

"flex-require": {
    "symfony/console": "*",
    "symfony/dotenv": "*",
    "symfony/framework-bundle": "*",
    "symfony/runtime": "*",
    "symfony/yaml": "*"
},

What does flex-require do? I cannot find it documented anywhere.