Knockouts May Require Trapping
In most cases, when two objects of different colors overlap they knockout — they won’t print on top of each other.
In the above illustration it looks as if a dark gray circle is printing on top of a yellow circle. However, in reality a gray circle and a yellow crescent shape are printed side by side The yellow knocks out leaving white space for the gray circle to print on.Because what you have are two touching colors there is a chance that the colors can get out of register on the press. Knockouts may require trapping to insure that no unsightly gaps are left behind.