Since AC8, objects have had the ability to have nodes that edit parameters other than height and width. In GDL jargon, this functionality is called Graphical Editing. These nodes respond to the stretch command as well as the stretch button of the pet palette. Even though they can do things besides stretch.
Since AC8.1, editing nodes have been diamond-shaped instead of round, so you can tell them from the non-editing ones.
In AC9, they decided that wasn't clear enough, and now editing nodes can have a different color. (Options -> Preferences -> Miscellaneous) The default is green, and I haven't been motivated to change it. When you see a green, diamond-shaped node, try it and see what it does.
Editing a crown miter
While editing with one of these nodes, a palette pops with the name and value of the parameter(s) being edited. (Some nodes edit two parameters at once.) If you type "N", the value in the palette will highlight and you can enter the value directly, just like typing "R".