Kinda cut and paste from the khorne daemonkin topic, but thought actually it probably needs an answer, or may have one somewhere else.
I am still unclear on this one and it has been argued in my gaming circle lots of times before. But can you choose to write down on your army list (or scrap of paper kept in sight of both players where it cant be tampered with) what unit is in what vehicle (AKA the guys with the red bases are in the rhino with the black X on the top, the guys with the orange bases are in the one with the black star of chaos, and the unit of chosen are in the blue star of chaos. Of course with open topped ones you could tell what is inside, but if it is sealed up, can your opponent force out of you what the contents is.
The argument against not declaring your stuff is inside of X is normally that you could change it. Though if you have an official list then that is hard to do unless your opponent is endlessly distracted and has to leave the room continually.
Or the other major one is that my army has high tech scanners or spies that surely would have been able to work out who is inside of what before this point in the battle. Especially if the things inside look nothing alike (Aka Kroot vs Tau).
We seem to have worked out that IF you choose to fire out of a firepoint it is normally clear what is inside shooting out, though even then that has caused issues such as "Well you know 4 boltguns shot out, and 1 of them I rolled with BS5, so that is as much as you know about what is inside" (then of course on the great reveal then it must be the lord and some guys with boltguns or whatever and that needed to be written on the list priory to this event, so you didn't just happen to have them there cos that was the best spot for them to be).
This has also spread to Fortifications and stuff as well and having to work out if people can see inside, and this is normally based on how open the building actually is. Though of course you could have your guys all pressed up against the walls or ducked down under the windows until it is time to reveal themselves.
This could also lead to ghosts though. Where a player could have bought several fortifications or transports but less than 100% of them have anyone inside them. So your opponent may really want to kill that Landraider thinking there is a big squad of death inside it, but actually the big squad of death is in actually in reserve for deepstrike. I know you are allowed to keep your reserves secret, as it actually says that in the book that you can "... conceal your true strength from the foe." But of course you need to have some form of written down option to bring them in I presume, or at least the option to in some way that is legal (though I have heard people planned a unit of daemons to outflank and they ended up deepstriking.)
Whats your opinion, or if there is a rule written down somewhere from on high, where is it located (hopefully not done by page number, just section and "between X and Y rules" or something)
