I don't know much about stocks trading, can someone ELI5? Thanks.
Fruit stand 1 is the cheapest, fruit stand 2 is the 2nd cheapest, and fruit stand 3 is the third cheapest.
You want to buy all the apples out there.
The rule is that you have to go to the cheapest fruit stand first, and exhaust their supply before moving onto the next cheapest fruit stand. If you want to go to ALL the fruit stands at the same time you need to tell them you are an 'OK' guy and are really buying fruit everywhere.
Well, someone deleted code and ignored one of the fruit stands. This violated a rule and cost the fruit buyer a lot of money in penalties.
In Europe and Asia, there are competitive exchanges, but no rule to force people to route there. There are some guidelines for brokers with customer orders to have a duty of "best execution", but for proprietary traders or professionals, they can do whatever they feel is ideal.
And guess what, there are very few "trade throughs" or cases where markets invert with each other despite lacking such a rule. People usually act in their economic best interest without a law mandating they do so. If it happens, it's an arbitrage that someone will eliminate very quickly.
Intermediaries make up 99.9% of the market because very few individuals would bother paying to connect to a dozen stock exchanges if the can pay a broker a few bucks to do their trade instead.
This rule seems designed to protect the consumer when the consumer and the stock brokers goals are not aligned.
If they weren't a BD, their BD (which is required to ultimately reach the markets) would be on the hook for letting this happen. ISOs are something that get a lot of compliance/regulatory scrutiny and most BDs don't let their customers use them. If they are allowed, there's a lot of reporting/transparency/surveillance to ensure they are issued properly. And as you can see from this fine, there are a lot of rules and corner cases one must cover and apparently even the best firms mess it up one way or another.
Latour's previous fine (read Matt Levine's continually excellent articles about this stuff) was for "Net Capital Requirements" which are designed to keep customer money safe, particularly in the presence of other customers who might blow up ... of course Latour has no customers, but it is still the rules. [Although IMO this one was a grey area where Latour probably had better models than what the rules required, but since they didn't match the rules properly they got fined.]