If you take a photo of a rectangle what you get is a trapezoid. The program will rectify the trapezoid so that it will be a rectangle again and the scale of the original object will be restored. This means equal lengths in reality will become equal lengths on the rectified photo.
You only need to specify 4 points in the photo which form a rectangle in reality. You can let the program render an even wider area as spanned through this 4 points.
It is also possible to erase areas in the rectified image. Simply draw over them in the program.
The program can recognize lines in the rectified image and save the lines as DXF file. So you are able to work further with the rectified drawing in you CAD application.
If you only need to convert scanned files into CAD drawings you and do not need to rectification at all you can use Paper/Picture 2 DXF.
This program is also available in German language.