From Xojo Documentation
Adds a separator line to the popup menu.
Windows and Linux do not support separators within PopupMenus, so this simply adds a row with the text "-" that can be selected by the user.
This method is deprecated for ComboBox, which does not support separators on any platform. This method will just add a row with the text "-" that can be selected by the user.
The following example adds a separator between some values: