FIX is why a fund's EMS can talk to thirty brokers without thirty bespoke integrations. Messages are numbered tags: 35 is message type, 54 is side, 38 is quantity, 44 is price, 11 is the client-order-id.
Sessions are sequenced and recoverable. If a connection drops mid-order, both sides resynchronise by sequence number and replay what was missed, which keeps order state from silently diverging.
Example: a new order is 35=D with 55=MSFT, 54=1, 38=5000, 40=2, 44=310.50. The venue replies 35=8 execution-reports: first an acknowledgement, then partials of 1,200 and 800, then a fill for the remainder — each carrying cumulative quantity and average-fill-price.
Related: execution-report, client-order-id, api-trading, order-management-system