For the sake of argument, TWOK-onwards Trek is erased from history. What's the difference between "screens" and "shields"? Are they part of the same system? A layered defense? The TOS/TMP defenses seem a bit more complicated than "press a button and a magic bubble pops up", but I can't for the life of me figure out how exactly to put it together. If there's some commonly-accepted fanon tech about this, I promise I won't shout at other posters (or the mods).