How selection works

I have class ContactAddress.xml in domains

<entity name="ContactAddress">
        <many-to-one name="contact" ref="Contact" required="true" />
        <string name="street" required="true" max="255" />
        <!--<string name="area" max="255" />-->
        <string name="village" />
        <string name="area" />
        <string name="region" selection=""  />
        <string name="country" selection=""/>

How to link country fields with cities so that when you select a specific country, only its cities come out?

<selection name="" id="">
    <option value='KGS'>Kyrgyzstan</option>
    <option value="KZ">Kazakhstan</option>

  <selection name="" id="">
    <option value="0" >Bishkek</option>
    <option value="1" >Osh</option>
    <option value="2" >Batken</option>
    <option value="3" >Chuy</option>
    <option value="4" >Jalal-Abad</option>
    <option value="5" >Naryn</option>
    <option value="6" >Osh</option>
    <option value="7" >Talas</option>
    <option value="8" >Issyk-Kul</option>
    <option value="9" >Almaty</option>

You can’t, you need 3 entities/domain for this :
Country (with string code, and string name)
City (with string code, and string name, string zip)

You need to create relationships between your lines
ContactAddress with :
many to one (country) to Country

many to one (city) to Country

now in your view, on the field, add a domain=« self.code == city.code » for example