Both types of CDC feathers have several different uses in fly tying. The larger feathers with long stems are, because of their structure, more versatile. They can be wrapped around the hook shank to form a fuzzy, segmented lifelike body on a dry fly. Two of the feathers can be placed together and used to form delicate wings on either caddis or mayfly imitations. Fibers can also be snipped from the stem and placed in either a dubbing loop or between strands of split thread. These are then twisted into a fuzzy dubbing rope and wound around the hook shank. The CDC helps the fly to float and adds a ton of natural motion to the pattern.
The smaller, shorter stemmed CDC puffs are most often used to form emergent wings as on an RS2. Or can be used to form both the emergent wing and a wing case, as on a Smoke Jumper.
These are just a few of the many uses for CDC feathers in fly tying.