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…

Strange but OK . Anyway the rules says :
NOTE: What is entered at Item 7 must match exactly what is entered in the Mode S Aircraft
Identification (also known as Flight ID) input device in the cockpit. If it does not, then the aircraft
will not be correlated with its stored flight plan and delays will ensue.

Item 7 is the Identification or callsign in the Flight plan Form .