Draw9patch tool, what for i should use red border, marked by pressing ctrl

In official documentation i didn't find answer. I can set border of image that indicates areas which will be stretched, but when i press ctrl key, color of border became red, and i don't understand what for it is. It looks like
enter image description here

Solution

This might be helpful: https://stackoverflow.com/a/11854891/1234007

Btw, this red is documented in 4.3 documentation as optical bound. Read about it here http://developer.android.com/about/versions/android-4.3.html#Graphics