Down loft, fill power
Down is the soft plumage of birds that lives under the feathers. A single piece of down is called a plume. The majority of down used in clothing and bedding comes from geese and ducks. Down is one of the best warmth to weight insulators available. It is not only warmer for weight, but compresses much smaller and is much more durable than most synthetic fill equivalents.
Loft is the volume for a given weight of down. High loft down is warmer for a given weight than low loft down. The most common measure of loft is fill power, a term which is often used interchangeably with loft. Fill power is the volume, in cubic inches, that an ounce of down will fill. Thus, one ounce of 650 loft down will fill 650in3.
Composition refers to the percentage of down versus feathers in a batch of down. In Australian and US regulation, 90/10 means that 90% of the batch is down plumes, the other 10% is small feathers. The higher this ratio, the lighter, softer and more compressible a sleeping bag will be. It also means there are fewer feather quills or spines to poke through the stitching or fabric of a bag.