A bleed margin is a part of your layout that is printed to the edge of a finished page.
Once a page is reproduced on press, the bleed margin is removed.
In PrintShop Mail, your total layout size (as seen in Size in the Layout menu) includes the bleed margin. To learn how to set bleed margins, see Imposition Settings.
When applying a Custom size to a layout and the bleed margin selected is at Page Level, the bleed is not displayed at the edge of the paper, but around the set of layouts.
Once you specified the Bleed Margin, check the "Crop marks" option and specify the Gap size, which is the distance between the crop marks and the edge of the document.