Some of the web fonts in the Typekit library are separated into multiple families, such as Proxima Nova and Proxima Nova Alt. These fonts were added to the Typekit library when web browsers were only beginning to explore CSS support for OpenType features, and so Typekit worked with the foundry to create different versions of the font family. This made it possible for users to access the small caps, lowercase numbers, and alternate characters in the font.
This limitation doesn’t typically exist in design applications, which support OpenType’s typographic features more reliably than web browsers, so alternate font families aren't needed for sync.
The alternate font families do not currently have a sync counterpart listed on the font family card or family page; if a font is available to sync, it will be listed on the main font family page. If you are using Proxima Nova Alt on the web, for instance, you will sync Proxima Nova and enable the Alt glyphs from the application’s OpenType panel.