A hammock underquilt is utilized as bottom insulation and allows you to hammock camp all seasons. Hammock underquilts are made with sleeping bag insulation that is suspended under the hammock where it can insulate the bottom of the hammock without being crushed under the hammockers weight.
If you have ever been winter hammock camping you know you can't survive without an underquilt.
You'll want to choose your hammock underquilt based on your winter hammock camping conditions. You can choose underquilts with higher fill power, they will be lighter and will be rated for colder conditions.
The highest available fill power may not be necessary if you aren't getting below freezing temperatures and if you don't need a super-lightweight underquilt.
A high fill power will provide more insulating ability for a given weight of down. The warmth of an underquilt depends not only on the fill power, but also on the quantity of fill.
So the difference between an underquilt with 550 fill power down and an underquilt with 700 fill power down is that the 700 fill power down underquilt will be lighter for the same warmth rating.
Down (or any insulation) provides warmth by trapping a layer of air that separates the cool side from the warm side. A thicker layer of trapped air allows for more insulation.
This thickness is often called "loft." An underquilt that uses 550 fill power down, for example, would have to use approximately 40%-50% greater weight of down than a similar underquilt that uses 800 fill power down to provide the same loft.