Creating a board game is pretty tough. There's a lot of rules you need to take into account, and you constantly need to take care that your game falls apart. With a game the size of Dungeons and Dragons, this is almost impossible! Luckily, the Wizards of the Coast managed to put together a game that works - as long as you follow some ground rules. If you break those ground rules with your homebrew, then the game can get REALLY messy. If you received this tag on your submission, then you probably used some game mechanic in a spot where it isn't supposed to be
When your homebrew gets declined with this reason and you wish to resubmit your content, please make sure that your content only uses rules that are similar to official content's rules.