Better description of margins for monochrome icon guidelines.

https://hig.kde.org/style/icon.html#margins

This is just an image, without an explanation for the “optional area” and “restricted area”. Questions are: When to use these areas, and why not otherwise?

Example I am imaging right now: Optional Area: May be used for icons which shall appear “cut of” somehow. E. g. an icon which only shows the top edge of a page symbol may use the bottom Optional Area.