Flarum uses the BBCode plugin of TextFormatter.
The Flarum BBCode extension enables that plugin and the following tags: B, I, U, S, URL, IMG, EMAIL, CODE, QUOTE, LIST, DEL, COLOR, CENTER, SIZE, *. Third-party extension might enable other tags.
You can find the documentation for the syntax of each of those tag in the TextFormatter documentation https://s9etextformatter.readthedocs.io/Plugins/BBCodes/Add_from_the_repository/
Note that the Markdown and BBCode extension don't enable the same features. Markdown has titles and spoilers for example, which are not enabled in Flarum's BBCode (but could be enabled by an extension).
