The event if fired only if some item's Tag is set, e.g.:

Private Sub fc_InitializeBox(ByVal sender As Object, ByVal e As MindFusion.FlowChartX.BoxEventArgs) Handles fc.InitializeBox
e.Box.Tag = fc.Boxes.Count
End Sub

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim xw As New XmlWriter(fc)
AddHandler xw.SerializeTag, AddressOf WriteTag
xw.Options.CustomTagSerialization = False
xw.Write("C:\test.xml")
End Sub

Private Sub WriteTag(ByVal sender As Object, ByVal e As MindFusion.FlowChartX.Xml.SerializeTagArgs)
If TypeOf e.Object Is Box Then
Dim b As Box = e.Object
e.Representation = b.Tag.ToString()
End If
End Sub

Stoyan