This has been covered elsewhere so try putting HK or Harmon etc into the search engine. It does seem to turn the stereo system into some kind of surround sound system with certain predefined sound parameters. The rear speakers are used more for bass. Also the speakers used are different. You get 8 speakers in the car rather than 6 for example.
Without HK it is more of a basic car stereo system.
'Boost' means you are getting a better quality in-car stereo than the basic. For instance the basic stereo is radio/cassette and 4x5 watts. Having boost means you get a better quality radio/cassette with 4x15 watts power and Dolby NR, RDS and auto-reverse added. Alternatively you can have Minidisk or CD but you have to have the Boost option anyway for these as they haven't produced a low cost equivalent that you could have.
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