It doesn't. "ship" refers to one instance of the "Ship" class. Each instance has its own copy of any properties defined on the class. So to answer willzi's original question: you use "_root.ship.enemies" because you want the enemies array that is part of that specific instance. If you tried to use "_root.Ship.enemies" it wouldn't work because the array isn't static, and if you made it work, that would mean you could only have one copy of the array in the whole applet. (That's probably ok for enemies, but imagine, say, health or position being the same for all ships.)
holy crap this game is all kinds of buggy... got the 'survivor' badge by dying on day 2; every time I "Retry" i stay dead; zombies that survive 5-6 clips of headshots. thpt. 1/5