Three realms, three hosts

The same authored form runs on different hosts by changing one line. A pure-logic Realm with no dependencies and millisecond Functions uses the in-process host:

name: levies
host: wasm
description: "Computes council levies from the rates types it ships."

A dependency-heavy realm — renders invoice PDFs with an npm library, needs real Node — takes the sandbox:

name: invoices
host: docker
description: "Renders and files invoice PDFs from billing signals."

And a host that does not exist — forward-looking, an illustration rather than a value you can write. Suppose a managed isolate pool: the platform ships the built bundle to a remote pool and fans dispatches out a thousand wide for burst work. The bundle is the same wasm artifact; nothing about the realm changes shape — a new host: value, and the platform grows an adapter:

# realm-translations/realm.yml — HYPOTHETICAL: no host implements `isolate`;
# a conforming host records a metadata problem and leaves the executable surface unavailable
name: translations
host: isolate
description: "Translates document batches on demand."

isolate is not part of this spec. It is here to state the test every proposed host must pass: if supporting it forces a realm author to change anything beyond host:, the design is wrong.