6th Shell
The 6th Shell is what contains the 5th Outcome and everything within, from the inside you can almost not tell it's there unless you really go searching.
From an outside perspective The 6th Shell looks like a spherical object, size varying. While it is durable in a special way, it can crack and be broken into and through. This would lead to the 7th Inbetween however.
The 6th Shell's size holds no great importance for a smaller shell can contain more fragments than a larger 6th Shell just as a Larger 6th Shell can contain more fragments than the smaller 6th Shell.
While it's speculative that the reason for the varying sizes is just to provide for the 5th Outcomes various and ever growing, shrinking, long or short possibilities, it's not actually known for sure. It should be noted that an entity within the 5th Outcome can't actually reach or touch the 6th Shell without first becoming a Player, which only a Player or a higher power can actually achieve the feat of moving out of the 6th Shell.
Touching the 6th Shell has an odd sense, as if something is pushing back at you, only, it can feel... inky, slimy, like a liquid or a solid, an Oobleck of touch perversing the senses.
Leaving the 6th Shell
The 6th Shell however is more like a safety blanket than a mysterious threat to any creatures existence, if a creature were to live on the 6th Shell or within, it would more closely be the 6th Shell themselves being sentient beings. One would soon find upon leaving the 6th Shell should they choose to ascend higher that perhaps the 6th Shell is a barrier for a reason. Afterall, getting this far does not garuntee your ability to go further.
Upon first leaving the 6th Shell, many would return due to looking at the 7th Inbetween, its not like the 5th Outcome which despite holding infinite possibilities is still 'contained', it's organized, the 6th Shell clearly shows its container. The 7th Inbetween however?.. It's Chaotic, its everywhere, you can't escape it, as if leaving the 6th Shell any further would mean potentially never being able to find your way back. Even if you were to take a single step and turn around, you may now have been eternally separated from the "cozy" home you might of made for yourself within the 5th Outcome, where the only walls and roof you needed was the 6th Shell.