The '+' means inner class. TextBlockComboBox is a inner class DataGridComboBoxColumn. According to the source code inside the WPF Toolkit, the TextBlockComboBox is a workaround of the bug in Selector.
Selector has a bug regarding DataContext change and SelectedItem property,
where if the SelectedItem due to old DataContext is a valid item in ItemsSource
but the SelectedItem due to new DataContext is not a valid item in ItemsSource,
the SelectedIndex remains that of old context instead of changing to -1.
This method is a workaround to that problem, since it is of high impact to DataGrid