Yeah levels are just a way that flashes uses to seperate up movies. If you are creating it using PHP and not from a designers point of view (IE FLASH) then i think you will always be accessing LEVEL 0
You can have as many level as you want... They will be stack on one to onother... very handy fonction! _level0 is the main movie and by definition hold most of the info (size, call to other movie...)
I was trying to figure out what target meant above so I checked out the libswf documentation (in the DOC.html file). It mentions two important targets: "_level0" and "_level1". "_level0" is for replacing the current movie and "_level1" is for overlaying ontop of the current movie.