So there's a question fairly deep in the thread asking me to define what I meant by the terms in the first paragraph. I'm reposting my answer up here to both clarify what I meant if it's not clear, and to invite those more knowledgable than myself to correct any ways in which I'm misusing the terminology or otherwise saying something untrue:

So electronic financial markets (whether ARCA/NYSE or Binance) have a number of ways that they can advantage certain participants at the expense of others. One of many is to make certain types of orders difficult or impossible for certain actors.
Broadly speaking lots of very conventional order types are "conditional" (limit orders are technically conditional), but various exchanges have at various times allowed the condition to effectively become "execute this order if I make money on it", which is a wealth transfer from those who can't place that order to those who can. "Displayed" or "displayed size" basically means that other market participants can see roughly "someone is offering to buy X amount at Y price, if I move quickly I can take them up on that". "Hidden" or "non-displayed" means that an order might execute in front of another but other participants can't see that before they act. "Non-displayed" isn't necessarily a bad thing either, but it creates scope for sophisticated participants to further set up advantages for themselves.

The "undocumented" part is the real killer: that's basically the idea that there's a secret API for playing with cheat codes that the exchange only makes accessible to certain actors. That's straight fucked up (and tends towards illegal as markets become more mature).

"Wash Trading" is roughly the idea that (typically) via intermediaries of one kind or another that an actor effectively trades with themselves. An actor might want to do this for several reasons, but a big one (maybe the main one) is to create the appearance of market activity where there isn't any legitimate commerce going on.

"Arbitrage" I think is technically defined as something like: "a transaction or transactions guaranteed to be profitable", but in practice the term gets applied more loosely than that. In the sense I meant: if gold is 100 quibbles in Foobarnia and 50 quibbles in Boofarnia, someone will buy a ton of gold in Boofarnia and ship it to Foobarnia and pocket the 50 quibbles, raising the price in the cheap place and lowering it in the expensive place and fairly quickly this gets you to 75 quibbles in both places (or whatever, there are transaction costs). There's an old quip: "you can shear a sheep many times, but you can skin him only once". If an arbitrageur has unique access to one or both markets, they can play the long game and just bleed profit out without actually providing the social utility of equalizing prices.

People do all this shit and more in practically every electronic market on Earth. It's quite a bit more regulated and monitored in mature markets like US equities and quite a bit more flagrant in e.g. crypto DeFi exchanges but how much net "rich connected people taking non-rich, non-connected people's money" goes on in one vs. the other is quite the controversy, as you can tell from the other comments in this thread.