There are two kinds of ease that are used in patterns. The first is Wearing Ease and the second is Design Ease. Design ease determines what is required in order for the garment to drape on the body as the designer intended.
All real-world garments must contain wearing ease to allow for movement and comfort. If patterns are drafted using only the model's measurements, the wearer couldn’t move around or sit down. Wearing ease is the number of inches or centimeters added to the actual model measurements to allow the wearer to live in the clothes. Wearing ease is generally less than one inch / 2.5 cm added to the measurements.
Design ease is added fullness beyond wearing ease incorporated into a pattern to create a specific silhouette, as dictated by the fashion designer. These variations are almost limitless, but manufacturers often designate several specific categories.
Real-world sewing patterns include a reference to design ease right on the package. Here’s a chart from Butterick, a real-world sewing pattern company, that shows you the large variance in fit.
There are five different categories of fit, from close fitting, which would be like a sloper with wearing ease, all the way up to very loose fitting. Just changing the design ease of a garment makes a huge difference in how it looks.
Regardless of the ease of the design, the ultimate goal is that the garment fits the person wearing it. Fashion designers have specific intentions on how the garment drapes, as well as comfort and style. Even though a design may include a dropped shoulder line or an over-sized bodice, the basis of the pattern always fits the intended model size.