On select field tag:
1.- In two places, when disabled form binding:
Whatever the value, the select is always multiple. When the attribute is informed with any value, the this specifies that multiple options can be selected.
There is no a standard, but usually is not defined when single selection and defined with multiple="multiple" when multiple selection.
2.- There is no possibility to mark an option as selected when disabled form binding.
- Provide the value that should be selected as another tag attribute
- Provide the name of a model attribute setted on the controller that should be selected as another tag attribute