The scope is where the variable exists. It must be a container. For example, In your case, you have 2 containers, Sequence and Do.
Now if you set the scope of a variable to the scope 'Do', it only will exist in that scope.
If you set it for Sequence, it will exist in all the main(this particular case).
if it doesn't appear to you the read/write scope, it's because you didn't change the sequence name.
You can change it in properties, it's called 'displayName'. After that, it will appear in the scope with that name.
hope this made things clearer