Most medicine programs require 3 letters of recommendation, one of which should be a departmental ("Chair") letter, but you should check the websites of the prelim programs to which you are applying to confirm.However, don't delay submitting your application if you are just waiting for your Chair letter. (I've gotten 2 prelim IM interview invitations so far, without a chair letter uploaded onto ERAS)
At my med school, the medicine chair letter is basically a form letter going over your performance in your medicine clerkship, NBME medicine shelf, and sub-i in medicine if completed. I happened to have worked with the attending who wrote my departmental letter during my medicine clerkship, but that was just a coincidence,
Also, the medicine chair/department letter (written by the department of internal medicine) is different than your Dean's letter, which is sent out by your med school (or ECFMG for FMG's) November 1.
