AlphanNumeric Characters for Mode S:Flight Plan Callsign

Hello to all. As ICAO Rules Says concerning filling a Flihtplan , aircraft’s crew should put into the field 7 (aircraft identification ) in Flight Plan , the 3 letters code of his company or the registration marking or a tactical callsign (if military) . In this Field 7 Crew must respect a maximum of 7 alphanuméric characters item : ex: DEVIL01. CReW have to put exactly the same callsign in the Mode s aircraft identetification . (Not more than 7 alphanumeric characters .( also Known as Flight ID into the input device in the cockpit) . MY QUESTION . Since few month i see on aircraft label some time a callsign with 8 charachters ??? how is that possible .???

ADSBx doesn’t deal with flight-plans. I’m not an expert on low-level transponder data formats, but I believe it’s possible for both ADS-B and Mode-S to transmit an 8-character callsign, and ADSBx will show whatever callsign was received by feeders. Whether or not it’s within ICAO rules is another matter…