This was the first and only game mode in Diep.io for a while, before Team DM was announced.
You can cross-team with others using signals, but friendly fire is enabled in FFA.
Teaming is often frowned upon by experienced players in FFA and Maze.
Some YouTubers own clans that are in  square brackets, and usually contain two letters, such as [MG] for the Master Gaming (also known as MasterOV) clan. These usually have a lot of teaming, and experienced players not in the clan are very likely to kill someone in that clan.