Source attribute in a `packer` block


I’m starting to use Packer. I’m working through the tutorial (see: Install Packer | Packer | HashiCorp Developer). The tutorial contains a sample template. The template include this block:

packer {
  required_plugins {
    docker = {
      version = ">= 0.0.7"
      source = ""

In this block the docker plugin is configured with a source attribute. Farther down the tutorial ask me not to worry about it:

The source attribute is only necessary when requiring a plugin outside the HashiCorp domain. You can find the full list of HashiCorp and community maintained builders plugins in the Packer Builders documentation page.

But, I would like to know what it is. Its value is, which looks like a URL, but is not. I’m assuming its some sort of namespace? This string isn’t mentioned in the documentation for the plugin. What is it for? If, in the future, I need a value for a source attribute, where could I find it?

AFAIU, this is a (artificial) namespace indeed and it is mainly used to distinguish HashiCorp-curated plugins from all other plugins. At packer init - Commands | Packer | HashiCorp Developer you can read this explanation:

HashiCorp does not officially verify third party Packer plugins, plugins not under the HashiCorp namespace hashicorp/*

which, IMO, should be dead explicit:

not under the HashiCorp namespace*