Can anyone explain how are inputs on the template website_sale.address set as required? I need to make zip required but cannot figure out what method is used. I don't see anything in template controller or I haven't found anything in js files.

<div t-attf-class="form-group #{error.get('phone') and 'o_has_error' or ''} col-lg-6" id="div_phone">

    <label class="col-form-label" for="phone">Phone</label>

    <input type="tel" name="phone" t-attf-class="form-control #{error.get('phone') and 'is-invalid' or ''}" t-att-value="'phone' in checkout and checkout['phone']" />

</div>

<t t-set='zip_city' t-value='country and [x for x in country.get_address_fields() if x in ["zip", "city"]] or ["city", "zip"]'/>

<t t-if="'zip' in zip_city and zip_city.index('zip') &lt; zip_city.index('city')">

    <div t-attf-class="form-group #{error.get('zip') and 'o_has_error' or ''} col-md-4 div_zip">

        <label class="col-form-label label-optional" for="zip">Zip Code</label>

        <input type="text" name="zip" t-attf-class="form-control #{error.get('zip') and 'is-invalid' or ''}" t-att-value="'zip' in checkout and checkout['zip']" />

    </div>

</t>

Phone input is required, zip is not. How is it done?