What is a documentation set?
A documentation set, or doc set, is a packaged collection of documentation files for a specific ArcGIS product, version, and language. In ArcGIS Documentation Center, doc sets appear as cards.
A doc set is downloaded from the Library tab and stored in the root folder on your machine or on a shared network drive. After a doc set is downloaded, you can open it from the Home tab to view its contents in your web browser.
Doc sets are generated from the same source content as their online-help output. The offline doc sets are deployed through a similar cloud-based system that the online help uses. This means that the content in a doc set is nearly the same as the content you would find in the online help for an ArcGIS product. It can also be updated at the same cadence as the online help, so you can expect to see updates to doc sets when there are updates to the online help. The main difference is that a doc set is stored locally on your machine, so you can access it even when you are not connected to the Internet.