So lets say you send an event to the engine, it processes it, and sends
it off to a connected application, and there is Operational Data on the
document.

The engine strips off the op-data (operational propery) on the Sub
channel, and when the event returns (as a status event, say) the engine
restores the op-data so that you can event upon it.

All this works shockingly well in the real world. (I.e. When it fails,
oy does it hurt! The Unix/Linux Bidir driver in earlier versions could
do this. Worked with the guys there to fix that! So it should not
happen again).

So here is my question. Where is that op-data based on event-id stored?
Is it in the TAO? I doubt it. Is it in memory?

The reason I ask is, if you restart ndsd, will you loose that info? Or
to ask my question more directly... What should I NOT do, if I really
do not want to loose that info?

Shim side, it is up to the shim as to how it stores it. Engine side, I
am not sure.