Custom property.
[Visual Basic 6.0] Property CustomProperty As Variant
[Visual Basic .NET] Public Property CustomProperty As Object
[C#] public object CustomProperty {get; set;}
[C++] public: __property Object* get_CustomProperty(); public: __property void set_CustomProperty(Object* );
CustomProperty is a place for developers to attach custom objects to an element. Previous versions of the software required that the specified variant implement IUnknown, but that requirement has since been removed.
This property is not used by the core ArcObjects functionality, it is strictly for developer use.