What's the meaning of "first-page-first order"?

Recently I prepared to submit to a conference of IEEE. In the requirements of submission, it indicated that:

PDF and Postscript files:

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  must not have Adobe Document Protection or Document Security enabled,

  
  


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  must have either 'US Letter' or 'A4' sized pages,

  
  


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  must be in first-page-first order, and

  
  


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  must have ALL FONTS embedded and subset.

  
  

I have searched for the phrase of "first-page-first", but still did not have an idea of it (sorry my first language is not english).

If anyone can tell me what's the meaning of it exactly I would be very grateful.