UserData property's value is not used by the XamSchedule in any way. It's meant to allow you to store any piece of data as part of the reminder object so that you can retrieve it later, for example when the reminder is activated. As an example, UserData can be a string that represents an action to take, for example a command to execute when the reminder is activated.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Server 2012, 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