Bonus level
From DoomWiki.org
A bonus level (or fun level in Doom 64) is an optional level with a normally out-of-range level slot. Unlike secret levels, they cannot be accessed through legitimate gameplay and can only be accessed using the idclev cheat code, the -warp launch argument, or through source port features such as the console.
As they are an unintended feature (at least in the vanilla game), bonus levels come with some quirks; only MAP33-MAP35 can be used in vanilla Doom II/Final Doom (due to MAP36 and beyond attempting to load invalid music tracks and therefore crashing the game), and finishing a bonus level will cause the game to crash. E5Mx can also be used to hold bonus levels in Ultimate Doom WADs, where they will use the Knee-Deep in the Dead sky texture.
Due to being unintended, bonus map slots were never used in any of the official PC Doom IWADs, however they are used in many community WADs, usually in vanilla WADs as a means of adding more levels than otherwise possible in vanilla. WADs for advanced source ports can also have bonus levels, and can feature them in slots other than the ones allowed in vanilla (including in MAP36+), as well as not having issues such as crashing upon completion, and can even make them accessible in normal gameplay via menus. However this is less common as the ability to have an unlimited number of levels in advanced source ports removes one of the main reasons for including bonus levels in your WADs. Similar to secret levels, mappers often base bonus levels around gimmicks that might otherwise not fit in with other levels in the WAD.
Evidence suggests that id Software were planning to include levels in slots MAP33 and above which is why they may be accessible at all. In some versions of Doom, the idclev cheat code is specifically hard-coded to allow you to access up to MAP40, eight more than Doom II or either of the Final Doom IWADs contain. The Ultimate Doom also contains placeholder "NEWLEVEL" level titles for the automap for E5Mx. The source code shows that this title is explicitly defined for each level, implying that an episode 5 was once considered for inclusion in The Ultimate Doom. The same title is also found in some Doom II prototypes for MAP28 and above. Romero's source data dump also contains unreleased maps for the slots MAP33, MAP34, MAP35, MAP36, and E5M1. The ports of Ultimate Doom and Doom II to Xbox contain additional secret levels in the slots E1M10 and MAP33, and as such they can be accessed in legitimate gameplay and can also be beaten without crashing the game. However both still use unintended music tracks ("I Sawed the Demons" and "Read Me While Listening to This") which can be heard when accessing these map slots as bonus levels in the vanilla PC version of Doom.
The Unity version of Doom does not support bonus levels, and attempting to load them will crash the game.
Only one official classic Doom game can be considered to include bonus levels, Doom 64, where they are called "fun levels" and can be accessed from the game's hidden level select menu. Like many community PC Doom WADs, three fun levels are included: MAP25: Cat And Mouse, MAP26: Hardcore, and MAP27: Playground.
