You can also use the Generate and Update OpenType Features dialog to automatically generate the feature(s) and accompanying lookup(s) for stylistic sets.
Within the OpenType Designer window click the first icon from the upper left corner. You will now be able to select which features you want to generate and/or update.
Stylistic Sets (ss01-ss20)
These allow for sets of stylistic variant glyphs corresponding to portions of the character set, e.g. multiple variants for lowercase letters in a Latin font. Be sure to name the variants with one of these suffixes: .ss01 .ss02 etc.