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It is an under-documented feature that one can specify a directory as the Ansible inventory source, not just a single file. In that case, Ansible merges the contents of flat files and any executable inventory plugins found in the directory. This is useful, for instance, to put localhost in your inventory for use with `local_action` even if your entire infrastructure is otherwise on EC2 or some other dynamic inventory source. I also use a flat file to create aliases for host groups automatically generated from the EC2 API, like "staging" for `tag_Environment_staging`.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.