Trading systems need prices for the liquidities that traders choose to trade. For that, it initiates a connection to the exchange to subscribe to liquidity updates.

The following diagram describes the communication between the initiator, which is the trading system, and the acceptor, which is the exchange:

The following diagram represents the FIX messages that are exchanged between the acceptor and the initiator:

Upon reception of these price updates, the trading system updates the books and will place orders based on a given signal.