Generally speaking one is an add on and the other is integrated into the design. Over the years the FAA has implemented progressively more stringent noise regulations both for aircraft that are flying as well as to the full certification process. The changes came in stages and have been progressively altered for the past 30 or so years.  [You can find some of the FAA legislation here][1] and the full text of the [relevant FAR's here][2]. As such newer engines will incorporate noise mitigation directly into the design as they need to pass the certification process. However in many cases it may be cost effective to design an attachment ([like a hush kit][3]) that can be added to an existing engine to comply with regulations since the engine was certified at a time that the regulations were far more relaxed. 


  [1]: https://www.faa.gov/c/environment_energy/noise/noise_standards.html
  [2]: https://www.ecfr.gov/cgi-bin/retrieveECFR?gp=&SID=848a71308ceac7a9cc4798db08de4dac&n=14y1.0.1.3.19&r=PART&ty=HTML
  [3]: https://en.wikipedia.org/wiki/Hush_kit