Putting sideloaded book in the Books category instead of Docs category

Hi there, could you please give me more details on the rules and formats that decide on whether a sideloaded book will show up in documents or books category? I have found out that in Calibre I should convert to mobi format having the [PDOC] tag replaced by the [EBOK] tag. But is doesnt seem to help my case. I also use Oasis 2, but cannot organize my books properly because of this. Thank you for your help!

Victor