Along with resolution, the threshold is the most important setting you need to choose when scanning a drawing for raster to vector conversion. Choosing the correct threshold can make the difference between raster to vector conversion success and failure.
Choose a threshold that is too low, and parts of your drawing will be missing from the scan.
Choose a threshold that is too high, and parts of your drawing will merge together.
In either event, valuable information is lost and you will not be able to produce a successful conversion.
You normally set the threshold at the time of scanning, using your scanner's scanning software. You may also be able to set a threshold to turn grayscale images to black and white in your raster to vector conversion software.