The "Department Chair Letter" is a misnomer. At US allopathic schools, it is never written by the dept chair. It is written by either the clerkship director or by a committee. It's a standardized letter, much like the MSPE but focusing only on IM rotations and experiences. It is essentially useless at DO schools or for IMG's, as usually they get a letter from a "chair" who knows nothing about them.
The classic example is SGU. Their "chair" letter is exactly word-for-word the same for every applicant.
IM programs should change their requirements to "a department summary letter if your school offers such"