The "slug" part is often used to denote any editorial or technical remarks, like project information, version numbers, date stamps, team members comments, etc. It's often used for collating certain information which needs to go along with the file. It's similar to that information part you see in technical drawings or blue prints.

Because it's a separate area, an extension to the page, you can decide to preview, export, or print pages with or without that slug, depending on what the viewer expects.

Oorspronkelijk geplaatst op Quora: What is Slug InDesign?"