Fill power (often abbreviated as FP) is a measure of the quality and insulating ability of down. It indicates how much space (in cubic inches) one ounce (approximately 28.35 grams) of down takes up under standardized conditions. The higher the fill power, the better the down's insulating ability.

Here are a few details:

1. Insulation : A higher fill power means the down can trap more air, resulting in better insulation. For example, down with a fill power of 700 has better insulation performance than one with a fill power of 500.

2. Lightness : High-quality down with high fill power is lighter because it can provide more warmth with less material. This is especially important for sleeping bags, down jackets, and other outdoor gear where weight is a factor.

3. Comfort : Sleeping bags or jackets with high fill power are often softer and more voluminous, which increases comfort.

In summary: Fill power is an important quality feature of down products, which provides information about their thermal performance and lightness.

"Cuin" stands for cubic inches per ounce and is a unit of measurement used to describe the fill power of down. It indicates how many cubic inches an ounce of down fills under standardized conditions.

The higher the Cuin value, the better the down's insulating properties. For example:

• 500 Cuin : Medium quality, offers good insulation but not as effective as higher quality down.
• 700 Cuin : High quality, excellent insulation and very light.
• 800+ Cuin : Top quality, exceptional insulation with minimal weight.

A higher Cuin value means that the down traps more air, resulting in better thermal performance and less material required, which is especially important in lightweight, high-quality products such as sleeping bags and jackets.