short answer: official record of what happened.

long answer: captain nog visits jake with some newly declassified materials from that era (think The Visitor setting, cozy house and all). The attack was staged, to “kill” Trip (which also explains the “bad acting” in the holographic version and any say inconsistencies). Trip was recruited by the precursor to section 31 introduced in S4, to go to Romulus and, iirc, delay/sabotage their warp 7 program. Best engineer in the fleet. Of course it’s only temporary, he’ll get back etc etc, but, he doesn’t. He spends most of the Romulan War surgically disguised as a Romulan scientist’s assistant (I’m going from memory here). Reed and Archer are the only ones that know initially, and T'Pol finds out through their strong psychic connection. Eventually the rest of the crew finds out one by one (spread out over the course of the Romulan War books). The lead-up and war changes the dynamic of the crew (i.e. Mayweather gets promoted to a new ship and has I believe three of them shot down from under him and is considered a bad omen). Without spoiling the ending of it, I was pleased with the ending. (and a couple of postwar novels since too, catching up on dangling threads like the Aliens from Silent Enemy and what was behind the self-repairing station from Dead Stop). Not everything works (maybe a little too on the nose on why TOS looks like that) but worth the reads.