The order settler is called by the transaction script when something is executed on the exchange. It calls Sway’s built-in log function to create a reciept that can be seen by an indexer. Below is something resembling the order settler code for Sail Exchange:
order_settler.sw
    contract;

    use std::logging::log;
    use order::{LimitOrder, OrderSettler};


    struct MakeOrder {
        id: u64,
        order: LimitOrder,
    }
    struct TakeOrder {
        id: u64,
        order: LimitOrder,
    }
    struct CancelOrder {
        id: u64,
        order: LimitOrder,
    }


    impl OrderSettler for Contract {
        fn take(id: u64, order: LimitOrder) {
            log(TakeOrder { id, order })
        }
        fn make(id: u64,order: LimitOrder) {
            log(MakeOrder { id, order })
        }
        fn cancel(id: u64, order: LimitOrder) {
            log(CancelOrder { id, order })
        }
    }