I said this in another thread, and I want to say it here to clear something up.
I don't think limbo is an alternate level. Limbo is a state of mind. When you have lost the fact that you are in a dream, you are in limbo. You have to be reminded that you are dreaming to come out of limbo.
So Mal and Cobb were on an ordinary (though very deep) level. But since they built from memories, and Mal got rid of her totem, she forgot they were dreaming, and put herself in a limbo state of mind.
When Saito died (he technically died on every level at the exact same moment), he would have normally gone from level 1 to awake. But since he was under the drugs, he got sent to the first, deepest level that had never been touched by any of the other dreamers. It was "limbo" for him because it was built entirely of his subconscious (ie, his memories), and it would have been impossible for him to recognize it was a dream. When Cobb got to that level, he could tell it was a dream because it was all built by Saito. So Saito was in limbo, but Cobb wasn't. Or maybe he was in a weak limbo state, but because he was there for such a short time before seeing both Saito and his totem, he was able to come out of it more quickly. He reminded Saito of where he was, and that brought him out of the limbo state of mind.
So there really aren't any "unknown rules" to limbo. The only difference is, because you are so deep, you have to understand you are dreaming when you die, otherwise you'd just go deeper. And, if you aren't certain what level you are on, like in Saito's case, you can't be for sure what the time difference is between earlier levels.
I don't think limbo is an alternate level. Limbo is a state of mind. When you have lost the fact that you are in a dream, you are in limbo. You have to be reminded that you are dreaming to come out of limbo.
So Mal and Cobb were on an ordinary (though very deep) level. But since they built from memories, and Mal got rid of her totem, she forgot they were dreaming, and put herself in a limbo state of mind.
When Saito died (he technically died on every level at the exact same moment), he would have normally gone from level 1 to awake. But since he was under the drugs, he got sent to the first, deepest level that had never been touched by any of the other dreamers. It was "limbo" for him because it was built entirely of his subconscious (ie, his memories), and it would have been impossible for him to recognize it was a dream. When Cobb got to that level, he could tell it was a dream because it was all built by Saito. So Saito was in limbo, but Cobb wasn't. Or maybe he was in a weak limbo state, but because he was there for such a short time before seeing both Saito and his totem, he was able to come out of it more quickly. He reminded Saito of where he was, and that brought him out of the limbo state of mind.
So there really aren't any "unknown rules" to limbo. The only difference is, because you are so deep, you have to understand you are dreaming when you die, otherwise you'd just go deeper. And, if you aren't certain what level you are on, like in Saito's case, you can't be for sure what the time difference is between earlier levels.