In addition to what Walter has written, I would point out that all of the rules for variable creation, assignment, and sharing have never been published. And these rules have undergone many changes over the years. You can often uncover what has happened with a particular line in a particular MATLAB version using format debug and/or a mex routine to give you some insight into the rules, but that's the best you can do. From a practical standpoint, you will never be able to discover all of these rules even for a particular version of MATLAB. And whatever you discover for one MATLAB version might not apply to a different version of MATLAB.