A subforum is a special type of forum that appeared under the main forum. They were mostly used for stories and roleplays, especially in a popular forum such as the Ninjago Forum. There were also some sub-subforums - subforums inside of a subforum. New subforums were added when roleplays grew and new forums were created for new themes. Subforums supposedly sped up moderation.
Sub-subforums referred to subforums within subforums. They appeared in the MLN Help Subforum, of which there were four. There was also once a subforum which housed two Mod Party topics in the Message Boards Forum known as the "Message Board Event And Party Forum", but it then received a sub-subforum for one of the topics, called called "MB Party Food And Stuff".