Disk Utility Zero Out Data option

I read that it can be good to write zeros to all sectors of a new external disk right away. The Zero Out Data option in the Disk Utility Erase operation states that it "writes zeros over all data on the disk". Does it actually write zeros over every sector on the disk or just sectors with data? The 7 Pass Erase option states that it "writes data over the entire disk seven times". I'm guessing this means all sectors get zeroed out whether they contain data or not. The different wording suggests that these operations do different things. Any one have clarification? Thank you.