What is bleed and slug ?

>>>what's "bleed and slug" and what does it use for ?

Read all about it in the InDesign Help files:

Specify printer's marks
When you prepare a document for printing, a number of marks are needed to
help the printer determine where to trim the paper, align separation films
when producing proofs, measure film for correct calibration and dot density,
and so on. Selecting any page-mark option expands the page boundaries to
accommodate printer's marks, bleed (the parts of text or objects that extend
past the page boundary to account for slight inaccuracy when trimming), or
slug area (an area outside the page and bleed that contains printer
instructions or job sign-off information).

If you are setting crop marks and want the artwork to contain a bleed or
slug area, make sure that you extend the artwork past the crop marks to
accommodate the bleed or slug. Also make sure that your media size is large
enough to contain the page and any printer's marks, bleeds, or the slug
area. If a document doesn't fit the media, you can control where items are
clipped by using the Page Position option in the Setup area of the Print
dialog box.


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Regards

John Waller