Exercise For The Reader Meaning

(by extension) A matter left to the addressee's judgement to decide.

Example: Used other than as an idiom: see exercise,‎ for,‎ the,‎ reader.
2003, Wells Earl Draughon, Advanced Writing: Fiction and Film, iUniverse (ISBN 9781469739816), page 167
  She does not ask to see the dossier on this man before she goes, even though the co-worker has said that he has seen it, etc. The credibility of this conduct is another matter and is left as an exercise for the reader.
2006, Hossein Bidgoli, Handbook of Information Security, Information Warfare, Social, Legal, and International Issues and Security Foundations, John Wiley & Sons (ISBN 9780470051207), page 413
  Moreover, determining what is “passionate,” “artistic,” and “suitable for young children” is left as an exercise for the reader.