"In printing, trap expresses the degree to which ink already printed on a substrate accepts another layer printed on top of it compared to how well the substrate (e.g., paper) accepts that ink.

(...)

When the two colors in question are spot colors, trapping is always needed: from the moment the artwork is imaged on film or plate, the colors are handled separately and are printed on two different printing units. The same approach applies if one of the colors is a spot color and the other a process color.

Trapping becomes more difficult if both colors are process colors and each is to be printed as a combination of the basic printing colors cyan, magenta, yellow and black. In this case, the trapping decision depends on the amount of ‘common’ color.

Another factor that influences the visibility of the traps is the direction of the trap. The decision as to which color should be spread or choked is usually based upon their relative luminance. The lighter (higher luminance) color is spread into the darker. This responds to the way the human eye perceives color: darker colors define shapes, therefore distortion of the lighter color results in less visible distortion overall. The lightness or darkness of a color is defined as its neutral density."

https://en.wikipedia.org/wiki/Trap_(printing)