This disc was given away by Camelot Software Planning (the makers of Shining the Holy Ark and Shining Force III), to anyone who sent in the spinecards ('obi') or tokens from all three Scenarios of Shining Force III (while stocks lasted).
The offer from Camelot was re-opened on several occasions, but the Premium Disc is still very sought-after. Due to the scarcity of Scenario 2, many people missed the chance during the final period of the offer. The disc includes extra battles (against the main bosses of most of the games in the series - in 3D!), a savegame creator for Scenario 3, a sound test, movies, interviews & adverts, artwork & 3D models. It's an amazing disc, and well worth picking up if you can find it.