See, that's the only part I don't get. Why would you ever want to erase just 800 sectors instead of all sectors on a disc? The only two reasons I can think of are (1) to make the disc unreadable, to protect private information and (2) to completely erase a disc that has no more than 800 sectors of data, like one or two small files.
Surely not to erase a disc that has over 800 sectors of data, as you would overwrite only some of the data and the rest of it would still be on the disc, requiring a full erase the next time you want to write new data to the disc.