The context item (could be an enumerator).
[Visual Basic .NET] Public Property ContextItem As Object
[C#] public object ContextItem {get; set;}
Product Availability
Available with ArcGIS Desktop.
Remarks
You need at least to write a minimalistic implementation of this property as shown here:
Private m_ContextItem as Variant
Private Property Let IContentsView_ContextItem(ByVal RHS As Variant)
m_ContextItem = RHS
End Property
Private Property Get IContentsView_ContextItem() As Variant
IContentsView_ContextItem = m_ContextItem
End Property
The ContextItem property will contain the item on which the user makes a right-click in the TOC and at this point a context menu may be shown.
At 9.0 a default context menu is shown when the user right-clicks in your custom TOC. If you need to get rid of this menu in order to implement your own you will find the Selectable Layers Tab example valuable.