What setting the migration up on a site gives it: the permission names its runs are gated by, admin:dashboard:migration and admin:api:migration, so a site's admin can be granted them. The names are shared by every site, so the first site's install creates them and every later one finds them; the system writes them, whoever asked, through MishkaCmsCore.Install.Bootstrap.ensure_permissions/1, as MishkaCmsCore.Runtime.ExtensionProvision does for every extension. The extension ships no components, pages or snippets.

The module is the extension's plugin on installer:extension:install (MishkaCmsCore.Install.Extensions): subscribing is what makes the extension installable, and :extra is what the installer's picker shows. It answers the payload whose key is "mishka_cms_migration" with install/2's report, appended to reports, and hands every other payload on untouched. Installing needs admin:dashboard:runtime:site; a site's admin installs it only on their own site. With no actor it runs only while the CMS itself is being installed.

The extension registers no settings, so it has no tab on the Settings page.