I have a family acquaintance who works at a fairly prominent hft shop in nyc. He's rather circumspect when I ask him about details of his job, which doesn't directly relate to trading but I get the impression he has to know quite a bit about how the various trade plans work, since he is deeply involved in maintaining the highly bespoke infra they run on.

After a few beers he'll sometimes start bringing up "customers purchasing order flow" from them, which apparently accounts for a significant part of their revenue. He's also indicated that this is not a practice unique to their shop but is fairly commonplace among companies that have highly advantageous (next-building or same-floor/dc equivalent latency) transport to various exchanges.

I assume, but am hesitant to ask directly, that this activity cannot actually be what it sounds like, but now I don't know.