We can email a drawing file to somebody maybe across country even. So that they could see the plans, they could markup and send back to us.
DWGSee creates overlays for markups, so that all the markups made by others will not infect the original drawing file. Markup overlays are saved in a separate *.MRK file. To save the bandwidth or email attachment size, the reviewer can just send back the *.MRK file and you can easily load it on your drawing in DWGSee to see the remarks...