So far, what I think I understand is:
It looks like "Global" objects exist across all layouts, with all their state information being globally consistent and available all the time, independent of any single layout. (e.g. Even their position in a layout will be preserved across layouts, as well as their layer number, (not name).)
It looks like objects with the "Persist" behavior, have their state information tied to their home layout, however that state info is saved on exiting the layout, and restored on reentering the layout.
Is that kind of what's going on?
Also, What happens if an object is "Global" and has the "Persist" behavior? Does that cause conflicts? Would there even be any point, or are Persist characteristics essentially subsumed by Global characteristics?
Other interesting stuff about globals I've come across:
Global or not, object instances placed in layouts don't seem to get spawned until the layout is executed, and so global object instances placed in a layout will not exist until that layout is first executed.
A global object instance, once created, and then destroyed, will not be recreated if you return to the layout that first spawned it. So, I guess the layout knows that it already spawned that global object instance and won't do it again until the game is restarted?
This seems to be the case even if in the editor, you place a global object instance in "Layout_A", then run "Layout_A" to spawn the placed global, then destroy it in "Layout_B", and then return to "Layout_A". The global object instance will stay dead.
Is there any other interesting stuff worth knowing about Global or Persist?