Events are intended to record the flow of information as discrete and standard units. The Source of the event will be a controller, usually a single car controller (sometimes referred to as a Slave Controller) or group supervisory controller (sometimes referred to as the Master Controller). Other types of controller may also act as a Source.

<xs:attribute name="Source" type="xs:string" use="required">
    <xs:annotation>
        <xs:documentation>Events are intended to record the flow of information as discrete and standard units. The Source of the event will be a controller, usually a single car controller (sometimes referred to as a Slave Controller) or group supervisory controller (sometimes referred to as the Master Controller). Other types of controller may also act as a Source.</xs:documentation>
    </xs:annotation>
</xs:attribute>