This is an image placer in layout that represents where an image is to go when it is printed. This placeholder doesn't print, but it fits the position and size of the image that will b e printed in that spot. This Keyline often is a rectangle with an x through it.
Optional mailer information printed in or above the address or in the lower left corner of the envelope. The information in a keyline identifies the mailpiece and its presort level. Under some postage payment systems, the keyline is a required line that contains specific information about the mailpiece.
A black lined area on a layout or art board indicating a precise area for the placement of a color or black and white reproduction. It is generally noted specifically to use the black line to "trap" the photo to the background or to drop the black line and simply use the keyline to create the window for a halftone placement.
A keyline, in graphic design, is a boundary line which separates color and monocromatic areas, or differently colored areas of printing on a given page or other printed piece. The line itself, usually consisting of a black (or other dark colored) border, provides an area in which lighter colors can be printed with slight variation in registration. In traditional paste-up graphics workflows, keylines for cropping were often merely indicated on original artwork, and then images were stripped into the area manually with the keylines themselves being added as part of the process.