Questions on customising the ribbon

'Callback for Combobox1 getItemID (NOT GIVEN BY CALLBACK GENERATOR)
Sub cmb_getItemID(control As IRibbonControl, index As Integer, ByRef ID)
End Sub

'Callback for Combobox1 getItemLabel
Sub cmb_getItemLabel(control As IRibbonControl, index As Integer, ByRef returnedVal)
End Sub

'Callback for Combobox1 getItemCount
Sub cmb_itemCount(control As IRibbonControl, ByRef returnedVal)
End Sub

'Callback for Combobox1 onChange
Sub cmb_onChange(control As IRibbonControl, text As String)
End Sub

Option Explicit
'There are two global variables at the top of a standard module
'iItemcount and vRngValues

Private Sub Workbook_Open()
'Counter variable
Dim x As Integer
'Counts the cells in the range, used later to set no. of Items in control.
iItemcount = Worksheets("Sheet1").Range("BoxList").Cells.Count - 1
'Re-Dimension the dynamic array to it's actual size.
ReDim vRngValues(iItemcount)
'Build the array
For x = 0 To iItemcount
    vRngValues(x) = Worksheets("Sheet1").Cells(1 + x, 1).Value
Next x
End Sub

'Callback for Combobox1 getItemCount
Sub cmb_itemCount(control As IRibbonControl, ByRef returnedVal)
'Use the global variable collected in the Workbook_Open()
'assigning it to the returnedVal parameter
    returnedVal = iItemcount
End Sub

'Callback for Combobox1 getItemID
Sub cmb_getItemID(control As IRibbonControl, index As Integer, ByRef ID)
'Set the ID parameter in the callback signature to each 'index'
'in the signature. We are adding cmbBox text to make it unique
'as using just a number may clash with other controls/items
'indexes in controls are zero based
    ID = "cmbBox" & index
End Sub

'Callback for Combobox1 getItemLabel
Sub cmb_getItemLabel(control As IRibbonControl, index As Integer, ByRef returnedVal)
'Sets the returnVal parameter, which carries the text back to the control
'This fires for each control 'item', so the 'index' parameter
'provides a zero based index for the current item, which we use in the code
    returnedVal = vRngValues(index)
End Sub

'Callback for Combobox1 onChange
Sub cmb_onChange(control As IRibbonControl, text As String)
'Set the 'text' parameter to a cell value
    Worksheets("Sheet1").Range("D1").Value = text
End Sub

<customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui">
    <ribbon startFromScratch="true"> 
        <tabs> 
            <tab id="myTab1" label="Demo Tab" visible="true">
                    <group id="Group1" label="Combobox" visible="true">
                    <comboBox id="Combobox1" label="Cell Link" getItemID="cmb_getItemID"
                        getItemLabel="cmb_getItemLabel" getItemCount="cmb_itemCount" onChange="cmb_onChange"/>
                    <labelControl id="myLabel" getLabel="lbl_getLabel"/>
                    </group>
            </tab>
        </tabs>
    </ribbon>
</customUI>