The Remote-Source plugin is used to retrieve project sources from a remote URL, and unpack them into a work directory (under /target by default). This is especially useful when the Maven POM represents an add-on build for an existing package that is normally built by Make or something similar.
This plugin actually consists of two mojos: resolve, get, unpack and relocate. The relocate mojo actually moves the project sources from a relative path under the unpack directory into ${basedir}.
The get mojo is bound to the initialize lifecycle phase, to enable retrieval of the project sources before the build is started.
The relocate mojo is not explicitly bound to any lifecycle phase, because it is currently an optional mojo which is only to be used when explicitly configured in the POM.