They are referring to the break away style of mirror as opposed to the bolt on style...two different mounts.There is also a two piece mirror and a three piece. The three piece mirrors are all bolt on. Some of the two piece are bolt on some are breakaway style. The only advantage I see is in a tip over the break away will not rip out large hunks of your precious bodyworks,it will however still give you lots of cracks to fix and repaint
