Marketing pretty much. Some vague cloud of "uses serverless to generate", "pushes static files to a CDN through an API", but if you drill down none of that is key to it. I.e. you'll find advocates state that if a site uses a static site generator or Netlify it's JAMStack.

It mostly concerns how often content is updated.

Real-time: dynamically generated on the server-side. (e.g. injecting the current user's username)

Occasionally: Jamstack (e.g. rebuilding a static site every time a contributors adds a blog post via the CMS)

Rarely: manual updates (e.g. manually deploying a new version of a small-town restaurant's website when the menu changes)

no, that's it. jamstack - static site generator, but javascript

Yea... SPA == same thing imho.