When and why to use &?, =?, @? in AngularJS?
The &, @, and = symbols are used to define the bindings (one-way, bi-directional, etc) for isolated scope objects, as you already know. Here is a pretty thorough tutorial on how all this works.
The ? symbol is used to indicate that the parent scope property to which the isolated scope binding refers to is optional. This means that if for some reason the parent scope property doesn't exist, then your application will continue to run without throwing the NON_ASSIGNABLE_MODEL_EXPRESSION exception.