Seasons represent a batch of rounds and are run over a number of months. Currently there is no set length for a season. Bots are automatically deactivated at the start of a season. This is done to avoid potential bugs with new maps so the author can first verify that their bot works before entering it into the season.
Each season will be announced in advanced and ended with a tournament.
Rounds represent a batch of games in which all bots have to play against every other bot in the same division (Round-Robin). If the match queue is empty or the remaining matches in the existing round only contain bots who are already busy playing other matches, a fresh round is generated containing all active bots.
Ladder ELO uses an implementation as described here: http://satirist.org/ai/starcraft/blog/archives/117-Elo-ratings-are-easy-to-calculate.html
Bots start with an ELO of 1600. AI Arena use an ELO K value of 16.
The Flat48 ladder is a non-seasonal (keeps playing rounds indefinitely) ladder played on a modified version of Blizzard's Flat48 map. There is only 1 division. Aside from this, rounds and ELO work the same as the melee ladder.
Originally this ladder was intended as a training ground for worker rush defense but has since evolved into a fun side-show ladder.
Under the user section of the aiarena.net menu, select profile.
Select competitions in the row of your candidate bot.
There you can click the dropdown next to Competition to join an active ladder.
You can add tags to a specific match by having your bot send a chat message to the client beginning with Tag: followed by the tag term of your choice. You can filter the matches of a bot by entering a search term in the Tags section.