The manifest.json file of an extension has a key "manifest_version" which for many years had the value of 2. You set the value of "manifest_version" to 3 to signify to the browser that your extension adopts the new API.
The name is very meaningful to extension developers. To others, perhaps not.
In this case, you can think of it as the API surface exposed to Chrome/FF extensions, or akin to the iOS version/Android API Level - it fundamentally affects what an extension can Do
Sure, extensions have manifests, but clearly the project is about much more. It seems like… an odd choice.