CAR 5.40 is the reference. The gotcha is that the person giving the ICUS must be designated by the operator to do this. Where I come from this is interpreted as a Training Captain who is training another person for command. A Line Captain may give the F/O the sector as PF, and he may make most of the decisions on that sector, but he is still the F/O.
The NZ notion of command practice could be something else.