min_to_receive_symbolare used by limit_order_create operation only and will be left to NULL for other operations.
exchange_rateis used by limit_order_create 2operation only and will be left to NULL for other operations
fill_or_killis the same as literally, either fill (match) or kill (close). That is to say that when the user puts an order, if it can be filled, it will be filled and if it cannot be filled, it will be closed. So fill_or_kill can also be counted as a means to cancel an order
expirationis to specify when the order will expire. It can even be any time within 28 days. The order is automatically canceled when it expires, so this is also another way to cancel an order.