Sight lines can also refer to the line which, if crossed, someone in the audience will be able to see you (as an actor, or techie). If flats are placed judiciously, then the sightlines offstage will be minimal, meaning that you can walk around backstage without worrying about whether you can be seen by the audience or not. However, due to some set designs, white lines have to be painted or taped on the floor to show actors where they can and can't go when not onstage (of course, the techies know this instinctively - they don't need the lines!)

If you're after any other theatre tech information, then do take a look at 'Everything you ever wanted to know about theatre tech, but were afraid to ask'