It's a giant dry bag to cover EVERYTHING in your non-waterproof pack. Works perfect, like all S2S products. Not much else to say; if you don't wanna spend like $500 on a completely waterproof pack that squeaks at you with every movement than get a regular pack and buy one of these for the same effect.

I bought a large (90L) Ultra-sil pack liner for a multi-week trek in coastal Alaska, where I'd need to haul huge loads of gear & food (76 lbs was my starting weight for this trek) and needed to keep it dry both hiking on land and packrafting across rivers & fjords.
Unfortunately, the bag tore wide open before it ever left my apartment. It was partially my own fault (I picked it up by its top seams while it had about 25 lbs inside), which is why I'm giving it 2 stars instead of 1. But in the end, I wasn't expecting it to be so frail. I used plastic trash-compactor bags for the trek instead, which did the job fine. In the future, I'll just use those and not worry about paying $40 for a pack-liner that doesn't seem to be any more durable.
I'd be happy to try 'em again, if they can make 'em a bit tougher. Maybe I just demand a bit more than some folks, but that's my $.02,

Good and durable pack liner. Its good to have extra protection to keep your gear dry. It seems the sizes listed are the amount it can hold once sealed up. But over pretty good fit for most packs.

This bag has performed excellent whether on a week long trip or a weekend escape. During a two hour thunderstorm on day two of six in the Sawtooths my tent bag snagged a branch and tore wide open. From then on my Sea to Summit bag pulled double duty as tent bag (by day) and food storage (at night) (I realize this may have not have been the smartest idea, seeing as it may cause the tent to smell like salami and sour patch kids). Everything stayed completely dry and the sack never took any damage despite some thick brush at times. It was quite a relief to set up a dry tent after 12 miles in soggy duds. The loop is great for clipping on a carabiner to hang the food. After a long trip I just machine wash it and pack it away for the next go around. I plan on getting another soon.