[DefaultValueAttribute(null)]
public string CustomData { get; set; }

<DefaultValueAttribute(Nothing)> _
Public Property CustomData As String
	Get
	Set

This property is serialized with other appointment properties. It isn't used by the scheduler, use it to keep your data associated with appointment. This property is serialized into xml and binary formats and it is saved into underlying data source if mapping for Appointment Properties is set. It isn't retained at export/import to iCal format.