Unit Existence: UnitExists vs UnitGUID

[stako]

Having a look at the Blizzard_ArenaUI addon, there's a line to check for the existence of a unit:

if ( UnitGUID(self.unit) ) then --Use UnitGUID instead of UnitExists in case the unit is a remote update.

What does it mean by 'remote update' here and what's the difference between using UnitExists and UnitGUID when checking for the existence of a unit?

[myrroddin]

UnitGUID requires the unit to actually be in the game, while UnitExists could be someone in the game or merely in remote chat. At least that's how I would interpret the intent. Also, UnitExists can sometimes fail when the player is in a loading screen, ie:// between worlds, whereas UnitGUID is assigned upon first logon and never changes until logout/exit.