Here is the ZONE.
This is supposed to show that perfect spectacle-free vision is a zone within which people don’t need glasses. It is trying to show that it is not a number like zero but a zone of focus within which people can see clearly without glasses or contacts. If the focus of your eyes falls within the grey area you are spectacle free. As your focus falls further and further away from the grey zone your spectacle requirement increases.
Example: someones focus is -0.6 myopic and +0.4 astigmatism they are highly unlikely to need glasses to see.
Example: focus is -1.2 myopic and -0.7 astigmatism they are very likely to want to wear glasses to see clearly.
The purpose for the laser treatment is to get the focus into the grey zone not necessarily on “zero.”