Variables from outside the function generally only enter a function's workspace when passed as input arguments to the function.
Variables from inside the function generally only leave the function's workspace to enter a different workspace when returned from the function as an output argument.
Functions generally can't access variables outside their own workspace.
[Those rules aren't technically complete, but when you're starting out they're a good mental model to use as a starting point for your MATLAB knowledge.]
In your first example you created the variable A inside so's workspace so the so function has access to the A variable. Since you didn't return A from so, A is not added to the workspace from which so was called when the so function returns.
In your second example you created A outside so's workspace and didn't pass it in as an input, so the so function doesn't have access to the A variable.
