Just thought I'd copy the relevant codes across from that link that @Noddy74 kindly provided below - for a quick reference:

That was a "Draft" document. I'm assuming that draft became official at some point.

The S in S2 stands for Special. I think the 2 might have something to do with the trade size but don't hold me to that.

The more relevant bit is the XT, which stands for Crossed Trade and happens when the buyer and seller are using the same broker. That's probably why the trade could go through before the open.

Everything you could possibly want to know about the exciting world of trading codes you'll find in this link.