These types of orders are sent as "crosses" where "citadel" reports they are both the "buyer" and "seller".

As an example, according to the NYSE FIX Spec, this is 54 (Side) = 8 (Cross).

This is how they are able to "internalise", by crossing retail flow vs their house account.

CATNMS https://www.catnmsplan.com/ helps with this.

I haven’t touched compliance in a long time. I tho they have to report to tape, not as a cross. They always have to report due to Manning rules.

Also, the trades were done prior to CAT. I suspect it’s from Blue Sheets.

Well, prior to CAT they had OATS, which has been around for a very long time, but was only "top level" messages (not a deep trace into the stack).

If you combine crosses with OATS you can still get some of the data you are looking for, just not as cleanly as with CAT.