When putting together books and magazines, there are three main measurements to be aware of:
- The Safe Zone, where it's okay to have text and any imagery that MUST MUST MUST be on the page no matter what.
- The Safe Zone is at least twice the space set aside between the Trim Line and the Full Bleed. Remember, the whole reason Full Bleed is not at the trim line is because that's how far the machine can deviate in either direction.
- The Trim Line, which is where the actual measurements of the book page come into play. But, because machines aren't always precise, there's...
- Full Bleed, which is how far out any images meant to go to the trim line should actually go.
- If an image only goes to the Trim Line, it's possible the machine could cut too generously and leave some unwanted color behind. Going all the way to the Full Bleed will cut off some of the image either way, but at least it looks cool.
Most noticeable in modern Comic Books, Manga, and even some Web Comics that dabble with this, either seriously (leaving ample margins between the edges of the page and any text / important elements, because they're Drawing For The Trade) or just because it looks cool.