Most people buy a down jacket to keep warm but there's a big difference between a down jacket designed for the high street and one aimed at keeping you warm in seriously cold temperatures.
This is the volume in cubic centimeters of a single gram of down when fully lofted - fluffed up. The higher the number the better. High loft down traps more air within the down jacket keeping you warmer.
The fill weight is the amount of down in your jacket. A jacket using low quality down can still be warmer than a jacket with top quality down if it has a high fill weight. But it will be a bulky jacket.
Most jackets use stitch-through construction where the outer fabric is stitched directly onto the inner lining, separating the down into baffles. This distributes the down evenly around the jacket but the down is pinched at the seams.
Fabric compartments are created to distribute the down around the jacket. This type of construction is very bulky but eliminates any cold spots for really cold conditions.