A Big Agnes sleeping bag is made a little different than most sleeping bags. The manufacture calls it a system. This bag has goose down filling on top and then a sleeve for the sleeping pad on the back. The system is so it eliminates cold spots, and allows for a more uniform body temperature which they call baffles.
Who really understands all this tech language? You just want your sleeping bag to be light weight, if you are backpacking. And warm when it is cold and dry when it is raining.
So how do you choose? First you need to know where you camp most In warm season will need a 30-40 degree bag. If you camp in 3 seasons (spring, summer, fall) you will want a 15-20 degree bag. And winter consider 0- -20 degree bag.
Then you have a mummy style or rectangular. The mummy fits closer to your body for more warmth, but the rectangular gives more wiggle room. Bags made of synthetic material can keep you dry even when wet. The Big Agnes sleeping bags have a water repellent finish.