From Joomla! Documentation
The onAfterExtensionInstall trigger occurs after the installation of an extension. It has two parameters:
A copy of the installer instance that was used. This it to get access to any error messages that might have occured.
The extension ID of the installed extension. The extension ID may be either a valid numeric value (positive integer), a zero value (when the extension doesn't have a corresponding entry in the extensions table but it is valid; a combined 1.5 language pack behaves this way) or false if the installation failed.
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.