Which three application contexts does TYPO3 ship with out of the box?

The application context is set via the TYPO3_CONTEXT environment variable and controls, among other things, how many error details are shown and what is logged. The three main contexts are Production, Development and Testing; if TYPO3_CONTEXT is not set, Production applies automatically. There is no separate main context "Staging" – for a staging server you use a sub-context such as Production/Staging, which behaves like Production but still allows its own settings. In the site configuration, for example, you can use baseVariants to define different domains per environment.