Well, they exchange unit information, which is unit history, composition (oh look, at some point a Warhawk in that assault star was replaced with something called a Iron Cheetah? what is that?), and what would be personnel files . . . which is really about what a codex is for the Clans. The point the information is exchanged is important too, it is immediately after the challenge but before the final determination of forces.
It is basically laying out all the information on the table, because if you hide something it is dishonorable. Use what I mentioned earlier, but say I just said it was a assault star . . . a star of Gargoyles will be factored into the final bid differently than a star comprised of two Dire Wolves, a Kingfisher, Executioner and a Iron Cheetah. Further, I do not have to tell them what a Iron Cheetah is, if they do not know that is information they could/should have had or could gain and so they are responsible for their lack of knowledge. But if I just said, 'assault star' then I am responsible for their lack of knowledge and it could be construed as dishonorable- not this does not mean you have to disclose loadouts.
Circle this back around to codexes. Say in 3066 as part of the Horses preparing for their invasion they send a force for a friendly trial against the Warden Wolves as part of the prep for material/resource sharing & support as referenced in the Cygnus & Hellstar fluff. Well, if I am the Horse commander and find out the 1st Strike Grenadiers will be the defending force . . . well, that instantly tells me some things. Star Colonel Hampton Schroeder is a bit of a special case. He was a Elemental and took so much damage physically he could no longer operate the battle armor. Schroeder underwent physical rebuilding and therapy along with retraining into a Mechwarrior- he finds the cockpits small and uncomfortable. Everything but that last, which could be assumed based on the physical measurements, would be revealed in his codex.
And that is a LOT of information for our hypothetical Horse commander to make a battle plan around.
1- Longer the Trial goes on, the more exhausted and frustrated the opposing commander might be
2- Expect the opposing commander to have a excellent understanding of Elemental tactics and use in combined arms- should be passed to subcommanders
3- Expect the opposing cluster to be well trained in Elemental tactics
4- Expect cluster's Elemental component to be extremely proficient, no disconnect for the MW CO over BA forces that is commonly found
5- As a former Wolf frontline Elemental during the Invasion he will have experience fighting masses of vehicles but very little in operating/using vehicles with mechs
6- I would know how often, based on his codex, he had fought the Horses or a comparable Clan force (Spirits/Bears IMO) and the results.
This means . . . as the Horse commander, I should not make plans to get extra advantages or a decisive victory from my Infantry/BA component. I would expect the Wolf commander to choose a battle ground that limits the usual HH integration of Elementals and armor with mechs in the expectation as a Horse commander I would be relying on those components.
To circle back around . . . there is a reason intelligence operations since Grog picked up a rock and bashed Mog on the head have sought to learn as much about the opposing commander as possible- why sports teams analyze tape about quarterback and coach tendencies in situations so they can manipulate the situations to put the QB/coach in what was proven the most disadvantageous position possible.