After you've cleared the game with Leon and defeated all of the Bosses, you can return to your save game and attempt to defeat the Boss Rush. To access this, enter the platform room.
2018, Ken Horowitz, The Sega Arcade Revolution: A History in 62 Games, McFarland (→ISBN), page 104:
He was so fond of the bosses the team created that he added a boss rush to the end of the game as a preamble to the final battle, a first for Sega games.
