I had the same question,once upon a time, and I e-mailed Marmot. After several months, I got the following answer :
Hi Kevin,
Sorry for the delayed response to your question…
In the Helium and your Hydrogen you can shift the down till the cows
come home. When you get below the 15 degree bags, any of them, we do
throw in a side baffle for free :-)
cheers,
Brian Rashap
Marmot and Dana Design Warranty Service Rep
And you know what? I can and do shift the down around as needed. A great feature for ultralight bags. I assume this applies for the Atom as well.