Algorithmic trading: Algorithmic trading records must be stored on an approved form accurate, time-sequenced records of all orders—whether placed, executed or cancelled. This rule applies not just to algorithmic trading, but to all trades; firms must keep for five years all the relevant data relating to all orders and transactions, whether for their own account or on behalf of clients.
This is an important requirement and much more complex than it may appear at first. The “time-sequenced” requirement is easily defeated by poor time synchronization or clocks that go backward (something that is still all too common). Imagine a trade comes into an exchange and is timestamped on entry and on exit but the computers creating the two timestamps are off enough to make the confirmation (exit) timestamp show the trade was executed before it arrived! Problem. Now scale up to a large distributed system spanning a number of colocation sites.