Hi the feature was just released and some changes may happen in future, but here's the concept of it:
1. User wants to buy from other users without price impact
2. Existing BUY/SELL limit orders are present on the order book
So the user can directly sell/buy from the limit orders. This excludes all AMM/LP orders so it's purely P2P. And since it does not cause a price impact it is OTC.
Yes it's different from how other exchanges are doing this, since we intend to hold 'unfulfilled' orders in the order book along with regular orders, and then it can be fulfilled either by a Limit order, or Market order, or an OTC order.
So, to understand it better: If I want to buy, let's say BNB, I can do it either through the usual way (by putting an order in the order book) or by putting an OTC order at the price I like/want and wait as I would wait if I make a Limit order. The difference is that my order won't affect the order book and the actual market price of the coin. Correct?
As for the "unfulfilled" orders, what do you mean? There are all orders at different prices (Limit), and all the exchangers keep them. Some, however, charge a fee so the traders can't keep them open until the end of time. So, basically, all the orders that are different from the Market price can be called "unfulfilled" unless the Market price "hits" them, and then they become "fulfilled".