For professional printing, global colours can be made to overprint. By applying an overprint colour to objects selectively you can control overprinting.
Overprinting means that you can print one ink colour on top of another instead of, by default, the underlying colour being 'knocked out' (removed). This prevents unwanted fringing being left around objects, e.g. text.
As a professional printing feature, overprint works when publishing PDFs using a CMYK colour space and PDF/X compatibility.
You don't need to explicitly make an overprint for black, for black text or black graphics, as this is set by default. On PDF publishing, you can control black overprinting using the Overprint black option in the Export Settings dialog (File>Export>PDF>More).
To create an overprint colour from scratch:
On the Swatches panel, select a Document palette from the palette pop-up menu. If no Document palette exists you can create one from the panel's Panel Preferences menu.
From Panel Preferences, select Add Global Colour.
Adjust the settings in the dialog.
Select the Overprint option.
To make an existing global colour overprint:
On the Swatches panel, -click the global colour swatch's thumbnail, then select Overprint.