I'll try not to lapse into EE on you.
I was just not understanding 'trade' grids as you seem to believe they exist. Traders set schedules and operations makes best efforts to follow. Operations balances the power on a link in real time, just as they have systems to do automatic generation control. The math is hairy, everybody knows that at the detail level, the grid is a chaotic system. Much as it looks scheduled at the hourly level.
There are times, when links are marginal that units are regulating the next region or three over. There is no reason that you can't. Traders just like to think all link flows are scheduled, tagged etc etc. What do you think happens when a big unit 'falls over'? All plans are in the crapper as the operations guys keep the lights on.
There are also units that physically exist in one region but are exclusively tied to the next region over. Usually this is the result of utilities gaming environmental regulations. (e.g. the both is and isn't a remaining coal fired plant in CA, depending on if you are talking about CA the state or CA the control area.)
Reality is also a little fuzzy on the distinction between generation/trading and distribution. Local cogen or small hydro for example was often small enough to generate at voltages you'd call distribution. To say nothing of rooftop PV (which is fair, they are so small, they just have to take the local utilities offer).
There is always a reconciliation process. Where 'as scheduled' is compared to 'as played out'. It's usually fairly friendly as the schedule is 99% accurate 99% of the time. Everybody involved has FERC (don't recall the Brit equivalent) rules hanging over them (e.g. 'take no actions that would compromise system stability').