What does the entityfield directive in the autocomplete map do?

[Guido1982]

If you had this in a FieldInfo map:

PHP Code:

<field>
            <fieldname>contact_id</fieldname>
            <features>
                <feature>
                    <name>searchmodule</name>
                    <value>Contacts</value>
                </feature>
                <feature>
                    <name>searchfields</name>
                    <values>
                        <value>
                            <module>Contacts</module>
                            <value>firstname,lastname</value>
                        </value>
                    </values>
                </feature>
                <feature>
                    <name>searchcondition</name>
                    <value>contains</value>
                </feature>
                <feature>
                    <name>entityfield</name>
                    <values>
                        <value>
                            <module>Contacts</module>
                            <value>firstname</value>
                        </value>
                    </values>
                </feature>
                <feature>
                    <name>showfields</name>
                    <values>
                        <value>
                            <module>Contacts</module>
                            <value>firstname,lastname</value>
                        </value>
                    </values>
                </feature>
                <feature>
                    <name>fillfields</name>
                    <values>
                        <value>
                            <module>Contacts</module>
                            <value>leadsource=leadsource,assigned_user_id=assigned_user_id,closingdate=support_end_date</value>
                        </value>
                    </values>
                </feature>
                <feature>
                    <name>mincharstosearch</name>
                    <value>1</value>
                </feature>
            </features>
        </field> 


What does the 'entityfield' directive do?