Magento 2: What is the source_service field element?

Question

In app/code/Magento/Config/etc/system.xsd, we can see that there is a source_service field element type that is defined:

<xs:element ref="source_service" />

Its definition is the following:

<xs:element name="source_service">
    <xs:annotation>
        <xs:documentation>
            Service used to populate select options
        </xs:documentation>
    </xs:annotation>
    <xs:complexType>
        <xs:simpleContent>
            <xs:extension base="xs:string">
                <xs:attribute name="idField" type="xs:string" use="optional" />
                <xs:attribute name="labelField" type="xs:string" use="optional" />
                <xs:attribute name="includeEmptyValueOption" type="xs:boolean" use="optional" />
            </xs:extension>
        </xs:simpleContent>
    </xs:complexType>
</xs:element>

However, I couldn't find any reference to system fields using this element.

What is this field for and how to use it ?

Answer 1

Well, It's been two years since you asked - and still nothing uses it. In fact, I can find no reference to the attribute in all of Magento except for the schema file.

My best guess is that it was intended to be a way to hook up a Repository to provide options for a select or multiselect field type. However, it was never implemented.