A dark blue carton has been trimmed after printing but has a small strip of white paper left over on the side of the carton. The background of the sleeve reaches to the edge as intended, however, the product name appears to be very close to a fold line. These errors are caused by improper use of the layout guides. That small strip of white could be because the file lacks proper bleed, or there is not enough. The text so close to the fold on the sleeve is probably because the designer didn’t check the safe area. The use of both guides protects artwork from the natural movements caused by printing, cutting, creasing and assembling.
To get an idea what the bleed and safe area does, consider a square and draw a line around it where the cut will be. Extend the graphic or color beyond the cut line, and then draw a line within the square. The part of the background or color that sticks beyond the line is the bleed. The area within the square is the safe area. Think of the cutter being a little off to one side of the line or the other. If you had extended the background or color past the line, the edge would be covered with a bit of color. Because that area is outside the cut line, it can be trimmed off. The square is the safe area for content, since you don’t know exactly where the knife is going to fall. This is the concept in a very rough format of how it can help you to imagine what a graphic and what text can do, rather than just try to memorize a concept.
Any artwork that extends the color background, photo, pattern, or visual content to the edge of the finished package requires bleed. It is not a border on the box that you see at the edge. It is removed by die cutting or trimming. A file in which the artwork ends at the cut line would require perfect alignment between printing and cutting, which is unrealistic. How much bleed you need depends on your production specification, so verify rather than assume it. Make sure the graphic or photo background covers the bleed, but not the logo, barcode or any text.
The safe area should contain content that must be complete and legible on the final product. The product name, the how-to section, the fine print, the logos and the barcode quiet zone should be well inside the cut edge. For folding cartons and sleeves, this also means accounting for crease lines, gluing flaps, glue lines, folds, seams, closure tabs, etc. A word might technically be in the safe area but it may get bent if a crease line is too close, or if it gets covered by a glue flap on a sleeve. A barcode may still be in the safe area but it may be more difficult to scan on a curved edge or close to a glue line.
Labels add another useful difference. A rectangular label might need bleed in all four directions, but the safe area must consider the shape of the container and how the label wraps. It may look a bit odd at the edge if there is a little shift on placement. For a wrap-around label, the overlap or seam may cover the print. The cartons are different, as separate panels will come together on the finished cartons. A graphic will look one way on the dieline layout, but if there are structural or production limitations, a graphic could be a little out of place on a corner.
Review the file before approval. Turn off decorative layers and review the dieline, bleed, safe area, creases and art. Verify that the graphic or color background has covered the bleed line, then that content in the safe area has been kept away from any creases or glue flaps. Print out a paper mockup and trim it a little off the edge, and crease it and glue or fold it. If the edge of the background still covers the edge, then your important content will be legible. This means your design works with the natural variations in production rather than relying on precise alignment.