Question

For some maven POM.xml, I found there are extensions in the build section. What’s the purpose to include build extensions?

Answer 1

Takes less than 6 second in Google and read. See here http://maven.apache.org/pom.html#Extensions

Extensions are a list of artifacts that are to be used in this build. They will be included in the running build’s classpath. They can enable extensions to the build process (such as add an ftp provider for the Wagon transport mechanism), as well as make plugins active which make changes to the build lifecycle. In short, extensions are artifacts that activated during build. The extensions do not have to actually do anything nor contain a Mojo. For this reason, extensions are excellent for specifying one out of multiple implementations of a common plugin interface.