What exactly is 'off world'?

Does the definition of 'off world' relate to sim boundaries in the NSEW directions only (for private island sims with no neighbours), or is there an upper and lower limit too?

I recenly placed a small mountain megaprim on my land and positioned it so that just the right amount of the mountain top was above the 20m water level. On panning my camera way down I saw that about 70m of the mountain was below the 0m level, ie, down at -70m.

Is the sim floor of 0m simply a reference point? Or can the space below 0m be utilised in any way (apart from hiding chunks of megaprims you don't need)?

Front

A prim's position is always its center coordinates. A huge prim may appear as not having its center in the center because it is often cut. You can't position a prim i.e. its center. below ground level. 'Off world' is only related to horizontal positions as far as I know.
See llSetStatus( STATUS_DIE_AT_EDGE, TRUE ); at
http://www.lslwiki.net/lslwiki/wakka.php?wakka=llSetStatus

Any prim with its center point below ground level is likely to be returned by the system.

Any prim above 4096m is forcefully returned.

A prim going too far from a sims borders is likely to go "off world" and thus is returned. physical prims like to do this kind of thing...

Also, when a prim moves into a different sim and the handoff times out, this can happen.