I use the control/non-control distinction.
An RTU collects data, analogs and/or discretes (sometimes via digital protocols like Modbus to local devices) and communicates those analog values and status conditions via a link to a SCADA or DCS. The link was traditionally termed 'telemetry' and can be wired, or wireless.
An RTU can also provide outputs, which are driven by instructions from the SCADA or DCS. An RTU doesn't make control decisions on its own, the mother SCADA or DCS or PLC makes the decisions.
A remote PLC that controls some process and is connected to other devices or a SCADA is not an RTU in my mind, it's a remote PLC, because the PLC's been programmed for control.
A remote rack of AI's kconnected back to a mother PLC is an RTU in my mind.
Of course, if my customer wants to call his remote PLC an RTU, he wins, as long as his check doesn't bounce.
Dan