This category is used to track redirects that are tagged with the {{Redirect category shell}} template or its shortcut, the {{Rcat shell}} template. Either no rcat has been added that would sort the redirect into a more specific category, or an editor has tagged a redirect with one or more rcats and has left the first parameter empty, for example:
{{Redirect category shell || {{R from/to/with...}}}}
{{Rcat shell || {{R from/to/with...}}}}
Editors who monitor this category may then find and help categorize those redirects.
Some redirects in this category may be test redirects used to check sandbox proposals.