Exposition of Data Representation

I would like to know how the data representation is exposed in slide 7 of information hiding:

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  Modifying an exposed data representation propagates to all code which directly accesses that representation

  
  
  
  
  • Perhaps the best example of the impact this can have is the Year 2000 problem
  
  
  • Legacy software for applications as diverse as nuclear power station control, air traffic control, finance, and the military were coded using exposed data representations
  
  
  • To ensure the software will work correctly come 2000 every single place that uses the date representation needed to be changed to store years using 4 digits rather than 2
  
  
  • The cost of this conversion has been estimated in billions of dollars!
  
  
  
  


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  Exposed data representation leads to change propagation ... affects maintenance costs ... big trouble

  
  

I would also really like to know whether information hiding means 'hiding the data using visibility identifiers(public,private)' or whether it means 'hiding data representation'.What actually does data representation mean?Could anyone help me.