The Market Data messages are used as the response to a Market Data Request message. In all cases, one Market Data message refers only to one Market Data Request. It can be used to transmit a 2-sided book of orders or list of quotes, a list of trades, index values, opening, closing, settlement, high, low, or VWAP prices, the trade volume or open interest for a security, or any combination of these.
Market Data messages sent as the result of a Market Data Request message will specify the appropriate MDReqID <262>. Unsolicited Market Data messages can be sent; in such cases, MDReqID <262> will not be present.
Market Data messages include many fields, and not all are required to be used. A firm may, at its option, choose to send the minimum fields required, or may choose to send more information, such as tick direction, tagging of best quotes, etc.