The minus sign indicates that subsequent selections with the marching ants will be subtracted from the first. There are three main ways to use the PS selection marquee - 1. select a rectangle - next go draws a new selection. 2. select a rectangle - next go adds to the first, overlapping it or not. 3. select a rectangle - next go subtracts from the first selection where they overlap. I'm now vague on the exact way you toggle these in PS7 but I suspect you will be able to hold down the mouse on the selection tool to get a choice.
David