plugins/maven-plugin/aggregator.html

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  <p>
  If checked, Jenkins will run individual module builds as separate builds. On a project
  with a large number of modules, or on a project where modules take a long time to build,
  setting this option to true may speed up the whole build, as different modules can be built
  in parallel.

  <p>
  Otherwise, leave it unchecked and Jenkins will build this maven project like you'd normally
  do from command line.

  <p>
  When your build uses "aggregator-style" multi-module aware mojos, you'd have to leave this
  option unchecked so that the mojo will have access to all of your modules.

  <p>
  Historically, this has been the default mode of the native maven project up until Jenkins 1.133.
  So if your project was created before that, you might want to uncheck this.   
  
</div>