That might be TY program dependent. BUt I didn't submit any letters from my sub I's. I had 1 FP, 1 path and 2 derm letters, the same letters that I used for my derm application fI used for my TY app. I did change my PS a little to include things like working in board varity of fields that would help me be a better dermatolgist (eg peds, surgery/plastic, burn unit, allergy/immuno, path rotations) and working with the team stuff.