He means that only 1 and 2* can be killed or wounded in battle without their parent unit being totally destroyed. 3 and 4* are excempt from that because the majority opion is that this could disrupt the chain of command to much (like when there is only on 3* is present in a region, commanding an army. If he gets killed or wounded, the army instantly dissolves and its corps become independent forces until the player can get another 3* or 4* there to form a new army).
There is merit to this thought, no doubt, but personally I think since the introduction of the the re-deployment rule this argument is no longer as convincing as it was before. Still, if Your army desintigrates due to death/wounding of the only eliglible commander on the spot, that iwould be quite bad, but IMHO realistic, too, and You could teleport a replacement pretty much always immediately in the next turn.
But this issue has been debated quite a few times already and participants could always only agree to disagree...
Regards, Henry

Henry D, also known as "Stauffenberg" @ Strategycon Interactive and formerly (un)known as "whatasillyname" @ Paradox Forums
"Rackers, wollt Ihr ewig leben?" (Rascals, Do You want to live forever?) - Frederick the Great, cursing at his fleeing Grenadiers at the battle of Kunersdorf
"Nee, Fritze, aber für fuffzehn Pfennije is' heute jenuch!" (No, Freddy, but for 15p let's call it a day!) - Retort of one passing Grenadier to the above
