Maintenance mode shows visitors a maintenance page while you work. Production mode is how a live Magento store should always run.
Maintenance mode
Turn it on before large changes and off afterwards. You can add your own IP addresses so you still see the store while visitors see the maintenance page. Deploys from the console handle maintenance on their own and only use it for setup:upgrade.
Maintenance mode returns an error page to search engines too. Keep it short.
Production mode
In production mode Magento serves compiled code and pre-built static files, which is much faster. Switching to production compiles the code and deploys the static content. Developer mode is only for development copies.
After installing an extension by SFTP in production mode, run setup:upgrade, Compile and the static content deploy from the Actions tab, then Flush all caches.
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