From Joomla! Documentation
The onBeforeExtensionInstall trigger occurs before the installation of an extension. It has the following parameters:
The method of installation that is occuring, either 'install' or 'discover_install'.
The type of extension that is being installed, for example 'component' or 'plugin'
A copy of the manifest of the extension about to be installed. This is only populated when the method is 'install'.
A copy of the extension table entry of the extension that is about to be installed. This is only populated when the method is 'discover_install'
Note: The extension adapter may shift internally to an update procedure from an install procedure. the same information is passed however the functional actions are different.