MACROMEDIA FLASH 8-ACTIONSCRIPT 2.0 LANGUAGE Reference page 920

You can think of the eight regions outside of the center (defined by the rectangle) as being like
a picture frame that has special rules applied to it when the movie clip is scaled.
When the
scale9Grid
scale normally, regardless of which regions of the scale9 grid they are located in; however, for
other types of objects the following rules apply:
All content in the center region scales normally.
Any content in the corners are only scaled when the center region is scaled to 0.
Any content in the top and bottom regions scale only horizontally. Content in the left-
hand and right-hand regions scale only vertically.
All fills (including bitmaps, video, and gradients) are stretched to fit their shapes, and all
fills (including bitmaps, video, and gradients) are stretched to fit their shapes.
If a movie clip is rotated, subsequent scaling is normal (and the
ignored).
For example, consider the following movie clip and a rectangle that is applied as the movie
clip's property:
scale9Grid
The movie clip.
When the movie clip is scaled or stretched, the objects within the rectangle scale normally, but
the objects outside of the rectangle scale according to the
Scaled to 75%:
Scaled to 50%:
920 ActionScript classes
property is set and a movie clip is scaled, all text and child movie clips
The red rectangle
shows the
scale9Grid
scale9Grid
.
scale9Grid
property is
rules: