It is not fired for custom items.
It can be the name of implemented function or explicit javascript statement(s). If the name of function is used, then that event handler may contain up to 8 parameters.
Notes:
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2