Where the paths come from depend on the implementation of the interface.  In this case, there's a model for each server type with a list of strings for fileset patterns. For example,  root directory ${config_dir}/some/path.   This will eventually be resolved to  ${config_dir:MyRuntime}/some/path   and then to  /home/rob/jboss/server/default/some/path.  

However, the original string ${config_dir}/some/path   can come from anywhere. It may be one of the initial paths provided by ASTools on a per-servertype basis, or it may even be contributed by the user themselves via UI and preference pages in the form of filesets (root dir with variables, includes patterns, excludes patterns, etc)

However, these details are hidden by the interface, and are not relevant to the interface's javadoc. All that is relevant to the interface javadoc is that a variable resolver is required (though one is set by default), and that the primary method getAbsolutePaths() will return IPath[] array with absolute file-system paths which point to existing files that this specific provider chooses to provide.