Crop Marks
To print an image without a blank area around the image, the non-printing areas must be trimmed after printing. Crop marks can be used to show the printer where the printing area ends, and the non-printing area begins. The part which is trimmed off is called bleed.
Crop marks will get a white offset, to see them better on dark(er) backgrounds.
The crop marks have a fixed length of 6 mm (recommended by the Ghent Workgroup)
Registration color
Crop marks are always in Registration color to ensure they are visible on all color plates.
Registration color means 100% of all process and spot colors, regardless of the colors that are used in the document.
See Wikipedia on Registration Black