Phases, e.g. not just active an neutral, depending on the area can be A, B or C phase to neutral, its generally down to the grid to balance load in an area across these phases, but if you have 2 or 3 phases run in difference circuits can be spread across different phases,
If the neutral or ground was to float in this situation, and overload on one circuit can trip 2 breakers, as the current is flowing back through a load on another circuit.