An interlock is a control that is wired so that when there is no demand for heating / cooling in a space, the heating / cooling generator and the associated pumps are switched off. Use of thermostatic radiator valves (TRVs) alone does not provide interlock.
Partial interlock means that the cooling and heating interlock controls are separate. However, the control function may be set up to minimise the possibility of simultaneous heating and cooling.
Total interlock means the controls are wired so that simultaneous heating and cooling is not possible. Where there are separate local heating or cooling units present, these will also need to be wired into the control system for total interlock to avoid the possibility of simultaneous heating and cooling.