More options for customizing a forum structureBy default, SMF can only handle top level categories and child boards. While this is sufficient for most setups, some would like a few more options to structure a forum layout.Initially, I had planned to scrap the board tree layout completely and go for a more advanced method that would allow unlimited nested categories and boards. However, this would have meant to either make the board structure incompatible with SMF or write a upgrade converter - neither of which I did like (yeah, I know, I'm a bit lazy sometimes, but you only got so much time when working on open source projects, so sometimes you need to decide where to spend the available time and where to save some of it).Sub-categories aka topic-less boardsThis is simple. It's a normal board which cannot have its own topics, but can still have an arbitrary number of child boards. Such a sub-category is styled differently in the board index and does therefore not look like a normal board row. Sub-categories make sense when you need a clear structure for a section of your forum, but don't want to put all the child boards into a top level category.Root categoriesA root category acts like any normal category except that it does not show its category title bar and is not collapseable. Boards inside root categories appear as they would be outside of all categories (hence the name root category). Again, there are some subtle styling differences to normal categories, but other than that, there are no functional differences. Technically, there is still only one level of categories and everything else are boards. Because of this, the board tree is fully compatible with SMF, but these two minor changes give you a little bit more flexibility in setting up your forum structure.