ACEP does not certify EM specialists, the American Board of Emergency Medicine does (ABEM).
ACEP is a political group, like AAEM.
A DO can be a member of AAEM, ACEP or ACOEP (like myself). Next year I will likely only be a member of AAEM.
If you are in a Osteopathic residency, you certify via ABOEM not ABEM.