It's like closed-source proprietary software. Installed by npm is a per-platform (linux-x64, osx-x64, win-x64, win-x86) binary execution file. The software license agreement is long and I'm not sure what is allowed.
Free for projects with up to 100 content pages. Pro pricing will be available for 5-10x lower than GitBook. Self hosting of the generated website, which isn't possible with GitBook. Ability to remove `Powered by` branding, which again isn't possible with GitBook. I guess those are the high level features. Hope this helps.