Hi Abduerhman,
I think the answer is none.
The dropdown search is a html + javascript widget, not a form control a.f.a.i.k.
All there is, is an onChange event, from where the application logic can pick up the selected value(s), any validations probably have to be done in the save flow, I suspect this widget falls completely outside the built-in validation mechanism of Outsystems.
Dorine