Identifier for custom action on client. That property will have priority over the Type or Act properties and will define the action which will occur on client. That property was designed to use when the type of action is custom and application process ClientSideEvents.BeforeAction with that value.
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