A cash shell is a company that does not quite meet the definition of a shell company, but whose main value nonetheless lies in its listing rather than its assets or its business.
Cash shells may be literally just that: a shell company with a bit of cash or investments added.
More commonly, shell companies have a rump of their former business. The normal fate of these businesses is to be sold once the shell successfully completes a reverse takeover.
As with shells, some exchanges dislike having too many listed cash shells, and may make it difficult or impossible for their listing to be maintained indefinitely.