The FAA amended 14 CFR 67.401 (Special Issuance of Medical certificates). Paragraph (j) now states that for a pilot who holds a Special Issuance Authorization letter or a Statement of Demonstrated Ability, that letter "...must be in that person's physical possession or readily accessible in the aircraft." The rule was implemented by the FAA Flight Standards Division primarily to be in compliance with ICAO standards. For pilots flying internationally this means that you may be asked to produce your Special Issuance Authorization letter (and any Statement of Demonstrated Ability) in addition to a current medical certificate.
Pilots with questions about this regulatory requirement should call their AME to discuss implications for their personal situation.