They appear in Apple's Human Interface Guide in Section
This widget appeared v0.13.
The command (which should open a dialog of some kind) to be run when the user clicks the button.
By default, this widget will use a standard arrows image, similar to the Disclosure Frame (dframe) widget. In most cases this will be sufficient,
however, there are some types of dialogs which have other standard conventions (eg, a dialog to choose a directory is often launched from a dialog with
a folder image, etc). This widget therefore allows for user customised images.
The imageName should have been previously created by the Tk Photo package or equivalent.
Available values: normal or disabled.
Force the widget to use Mac style, even if the OS platform isn't a Mac.
By convention, images with this widget should be 10 pixels high and (usually but not always) 10 pixels in width. Colours: greyscale and dark coloured, with a transparent background.
This reflects the history of this button class in the Mac OS range before OS X.