I have the same computer and that tripped me up at first to but as people have commented before me, that is the "odometer" for your bike. If it is a new bike set it to 0, if it is a used bike and you know the mileage this is where it belongs so you can track the number of miles you've put on the bike from that point forward.

I actually have the BC 1400 and man does that thing drain batteries, 6 months is all I got out of the batteries in my computer.