When you run Maven 2 with the -X flag, and you watch as it configures plugins, you might see output like this:
[DEBUG] Configuring mojo 'org.apache.maven.plugins:maven-clean-plugin:2.3:clean' -->
[DEBUG] (f) directory = e:\projects\foobar\target
[DEBUG] (f) excludeDefaultDirectories = false
[DEBUG] (f) failOnError = true
[DEBUG] (s) directory = .
[DEBUG] (s) includes = [**/*~]
[DEBUG] (f) filesets = [file set: . (included: [**/*~], excluded: [])]
[DEBUG] (f) followSymLinks = false
[DEBUG] (f) outputDirectory = e:\projects\foobar\target\classes
[DEBUG] (f) project = MavenProject: foobar:foobar:1.0-SNAPSHOT @ e:\projects\foobar\pom.xml
[DEBUG] (f) reportDirectory = e:\projects\foobar\target\site
[DEBUG] (f) skip = false
[DEBUG] (f) testOutputDirectory = e:\projects\foobar\target\test-classes
[DEBUG] -- end configuration --
What is the difference between an (f) and an (s)?