angular directive - When and why to use &?, =?, @? in AngularJS? -

i follow tutorials make angular directives. in isolate scope, tutorials define scope like:

scope: {    model: '=?',    data:  '@?'  } 

and tutorials define scope without question mark like:

scope: {   model: '=',   data:  '@' } 

can explain me difference or purpose of these proper example. thank you.

the &, @, , = symbols used define bindings (one-way, bi-directional, etc) isolated scope objects, know. here pretty thorough tutorial on how works.

the ? symbol used indicate parent scope property isolated scope binding refers optional. means if reason parent scope property doesn't exist, application continue run without throwing non_assignable_model_expression exception.