Measure Shape >> above best fit plane

[Toshi_OK]

Hi,

I have small question about "Measure Shape" part.

What does it means "Above Best Fit Plane"?

On the help file, it says " Best fit and mean level planes are calculated based on the drawn polygon vertices. ".
does it means PhotoScan finds flat plane from the model and set it as the base altitude?

Could someone let me know how I can make good use it for?

Thanks in advance!

Toshi