Yes I've contributed a lot to hive engine. It's in JavaScript (nodejs), backed by a MongoDB database.
App beneficiary tax is a config tribe owners can use to add a automatic beneficiary to post payouts for any posts not created through their designated app. It's something leofinance, stemgeeks and neoxiancity use for instance.
Oh! so it's like if I use POB's front end, but I use LeoFinance's tag, leofinance charges a tax on not using their front end?
Yes that's it. This is a feature on the old scotbot tribes which I recently added to the comments contract on hive engine, which will allow tribes waiting on that feature to transition over. It's likely that what I said might not make a whole lot of sense... it's probably time for me to finally write a more detailed post about the current state of tribes which I've been putting off.