This is a list of currently available tags (it can be extended later):

abuse-extended      Abuse by extended music and ignorance of native audio loops support (by OGG, FLAC, and Opus files), and leading to the serious bloatware.
battle              Levels designed for the battle mode.
bloatware           FXXXXNG GIGABYTE. FXXXXNG GIGABYTE FOR A DEMO?! Actual bloatware for no real reason, refuse to use audio looping (FLAC supports that!) and compressions even lossless FLAC.
bossrush            A marathon of boss battles.
collect-a-thon      Every level (or almost) contains many stars to collect.
collection          This package is a collection of multiple episodes or levels.
contest             This is an episode built from levels submitted during a contest.
credits             Author explicitly requests to credit when use this content.
cutscene            A level that implements some (semi-)automated scenario that player just watches and has the minimal interaction.
easy                An episode suggested to newbies as a good start.
hardcore            A hard level/episode suggested to experienced players who likes a lot of challenges.
heavy               Episode has unusually large archive size, so, it will be downloaded for a while.
hidden              For some reasons, this entry is not supposed to be widely visible for everyone. Primarily for author's request. The explanation note is required to add this tag.
horror              The level/episode contains spooky/creepy/scarying content. Play this on your own risk.
hot-garbage         Creation of the very bad quality.
hub                 A hub-based episode (uses a large level with many of doors to enter other levels instead of the world map).
intro               An intro level that can be placed at the root of the SMBX resources or TheXTech's game assets pack to be played while main menu is open.
introset            A collection of intro levels that gets played randomly every time when entering main menu.
joke                The level/episode developed to make a fun rather than something serious. But in some cases this can be combined!
kaizo               Extremely hard level/episode that requires to perform precise tricks to complete it, any small mistake is fatal.
legendary           Very notable level/episode in the community through long time. Nobody can just set this badge, it's for the chosen!
long                A large and long episode/level.
made-by-newbie      Creation of beginner/low-skilled designer.
made-by-thief       Episode built with levels taken without permission of their authors and without giving credits, and even more, claimed as "made by me".
minisode            A small and short episode.
multilang           This level/episode is translated into multiple languages, or has different editions in other languages.
no-credits          Author explicitly allows to don't give credits.
no-derivatives      Author explicitly forbids making derivative works and/or re-use the content.
nonenglish          The original language of this level/episode is different from English.
normal              Normal episode that doesn't have unique things compared to others.
offensive           The level/episode contains rude/offensive content. Play this on your own risk.
outro               An outro level that gets played every time when completing an episode and triggering the "end game" event.
port                This level/episode is a port from the different engine.
puzzle              Solve various riddles and logical challenges.
remake              This level/episode is a remake of another game or episode.
short               A short episode/level.
stars               Episode contains stars/leeks/etc.