Here is a brief explanation that I posted on this thread about a week ago. Hopefully it will explain the subtle, yet important differences.
Blanking is a feature that I'm used to, having owned a CRT projector, but is not common in LCD projectors. Generally when I set up my image on my screen, I want to start with no overscan, and then add about 2%. This will take care of virtually any overscan "garbage" that may appear at the top or bottom of the screen.
Generally, I have found that HDTV does not require any overscan, but older types of video still do.
There are two ways to accomplish getting to 2% overscan:
1) For most front projectors (including the 6800) , there is an "overscan" setting, which can be set to any value between around 90% up to 100% The problem is, in order to do this, the projector re-scales the pixel mapping, which can slightly soften the picture (e.g. no pixel perfection). The image does not shrink; it is simply re-scaled.
2) The 6800 allows me to zoom the screen where there is a little overshoot on all 4 sides of the screen, with no overscan adjustment. Now this would really be annoying if I left it that way, however with blanking I literally shut off (or "blank" out) all 4 sides of the picture, one side at a time, until the image is perfectly framed within my screen.
No overscan adjustment, no softening!