When you modify a map's zoom, icons are either drawn enlarged or shrunken relative to their native size. Now, picture a screen object -- for example, a screen border having the same dimensions as the map control -- that anchors to the edge of the map control. This object becomes practically unusable once zoom is changed (too large if zoom is increased, in my case).
appearance_flags = RESET_ZOOM
This object would ignore the map's "zoom" setting when drawn.