Unlike previous 3v3 entries, KOF '99 introduced 4v4 teams where the fourth member acts as a Striker —a support character called in with a button press to disrupt or extend combos.
Playing a "Plus" ROM typically requires a Neo Geo or arcade emulator: Kof 99 Plus Rom
In the standard arcade version, icons like Kyo Kusanagi and Iori Yagami are hidden and require specific cheat codes or high "Battle Ability" points to encounter. The Plus ROM typically makes these characters immediately selectable on the main screen. Unlike previous 3v3 entries, KOF '99 introduced 4v4
To understand why the Plus ROM is sought after, it helps to know the unique (and sometimes divisive) features of the base game: Unlike previous 3v3 entries