Some examples of hidden information that doesn't generally get disseminated in exchange orderbook APIs:

* Attribution: who's making the order?

* Short labelling... are they selling or shorting?

* Non-display or iceberg orders (not common in crypto?)

* Immediate-or-cancel orders... the executions hit the feed, but not the original order details. Also whiffs (order but no fill) don't get disseminated in any way.

* Certain order types that may rest on the exchanges order book but either don't have a specific price or display doesn't make sense... market orders, midpoint orders, pegged orders, auction order books (less common in crypto)

EDIT: On the attribution side -- they could also know the leverage any customer is taking and use that adversarially (which was the straw that broke their camel).