MASingleOptionDescription with add button

Hello All, 

i have one description based on MASingleOptionDescription:

linkDescription
	^(MASingleOptionDescription new)
		reference: MANazionalityModel description;
		selectorAccessor: 'nazione';

		options: [MANazionalityModel DataBase asArray] asDynamicObject ;

		label: 'Nazione';
		beSorted;
		beRequired; 
		beEditable;
		beSearchable; 
		priority: 40;
		yourself

Now my questions are:

1) In the rendering of it, where i can select one option,

    i can manage add buttons to add new element to options ?

2) reference: MANazionalityModel description

    What's the sense of this declaration?

    When define MANazionalityModel descriptionWithOnlyOneDescription the system d'ont answer well  ?

Thanks,

Dario