vcpkg registries version and port_version

Question

On the documentation on creating vcpkg registries I see

{
  "versions": [
    {
      "version": "2.6.3",
      "port-version": 0,
      "path": "$/ports/kitten/2.6.3_0"
    },
    {
      "version": "2.6.2",
      "port-version": 0,
      "path": "$/ports/kitten/2.6.2_0"
    }
  ]
}

Why do we need port_version, isn't version enough?

Answer 1

From the documentation:

Package versioning information is divided in two parts: a version string and a port version. Port versions are a concept exclusive to vcpkg, they do not form part of a package’s upstream. But allow for versioning of the vcpkg ports themselves.

In other words, it's used to keep track of changes in the vcpkg package of the dependency, rather than 'real' changes upstream.

For example, imagine there is a dependency missing from the vcpkg manifest. Adding it to the manifest would increment the port version but not the version of the software that is being provided.