Is Ghost's import ZIP format documented anywhere?

Note: I’m not asking for help. I’m asking about documentation.

As I’ve continued getting my custom Blogger migration scripts working, I’ve repeatedly bumped into constraints that have differed from existing examples, and that don’t seem to be supported or refuted by what seems to be the relevant Ghost documentation.

In particular, neither of the custom migration pages I’ve found even mention ZIP files at all (Importing content from other platforms to Ghost or Migrating to Ghost - Developer Guide)

The first I learned of it was from reading the medium_to_ghost code on GitHub, from which I inherited some format patterns that caused me issues down the line:

I eventually discovered that the format is at least somewhat documented on the Ghost-to-Ghost migration page (How to migrate data from Ghost to Ghost), but that’s also not complete — it doesn’t mention that the ZIP file can contain JSON, for instance.

I’m perfectly capable of reading the Ghost source code and figuring things out that way, but it’s slow. Is there more complete documentation anywhere around what file formats Ghost can import?