Those three icons for plug-ins

This has puzzled me for a while, ever since FlightCrew became a plug-in. On the right of the "visual" menu bar, there are three icons that can be associated with various plug-ins. What are they supposed to represent? The closest I can come is an oldfashioned flat-iron of the type my mother used to heat on the stove prior to ironing.

Thanks for any enlightenment!