If you happen to complete all 300 numbered Missions, you can obtain the Judgemaster Cid in your Party Arsenal. Being this, you can now go along with the Judgemaster Cid and then you will unlock 25 Judge Missions. To start the Judgemissions, you have to have completed all 300 Regular Game Missions. Then, visit Bervenia Palace, where a scene containing Cid will appear and then Cid will join your party, unlocking the Judge Missions.