What is the use of defining require field in a composer.json?

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“require”: {
“php”: “~5.5.0|~5.6.0|~7.0.0”,
“magento/module-config”: “100.0.",
“magento/module-store”: "100.0.”,
“magento/module-page-cache”: “100.0.",
“magento/module-directory”: "100.0.”,
“magento/module-backend”: “100.0.",
“magento/framework”: "100.0.”
},

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The first (and often only) thing you specify in composer.json is the require key. You’re simply telling Composer which packages your project depends on.

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