ToolbarItems is the collection within this class that contains all the components that make up the ToolPanel. This collection is comprised of objects derived from ToolbarItemBase. 

Autodesk.AutoCAD.Customization.ElementBase
    Autodesk.AutoCAD.Customization.VersionableElement
        Autodesk.AutoCAD.Customization.CustomizationElement
        Autodesk.AutoCAD.Customization.ICustomizationContainer
            Autodesk.AutoCAD.Customization.Toolbar
                Autodesk.AutoCAD.Customization.ToolPanel

Public Class ToolPanel
Inherits Toolbar
Inherits ICustomizationContainer

public class ToolPanel : Toolbar, ICustomizationContainer;