For vertical display use the additional guidance in brackets widget="RadioSelect[direction=vertical]"
The case of typical use of this widget is to reference a record belonging to particular entities. For example, to associate an email address to a client or user (entities Customer and User).
Example:
<entity name="Email">
...
<string name="ref" selection="selection.email.ref" />
<integer name="refId" />
...
</entity>
Two fields are required, a string for the type of the referenced entity and an integer for the id of the referenced record.
Recommended for the entire field naming convention is: <nomDuChampReference> Id .
<selection name="selection.email.ref">
<option value="com.axelor.contact.db.Contact">Contact</option>
<option value="com.axelor.auth.db.User">User</option>
</selection>
<form title="...">
...
<field name="ref" title="Reference" widget="RefSelect[related=refId]"/>
...
</form>
The attribute related widget is optional if the name of the entire field used for this purpose in the entity following the recommended naming convention.
The attribute selection can be redefined in the view.