Definition for "logical difference"?
See Date's book:

Date on Database: Writings 2000-2006

Chapter 4: On the Notion of Logical Difference

Salient quote:


  you'll be aware that I often appeal in my writings to the notion of
  logical difference. That notion is one I find extraordinarily useful
  in my own work; it's a great aid to clear and precise thinking… The
  intent of what follows [in chapter 4 of this book], then, is to offer
  a brief introduction to the concept of logical difference.


…bearing in mind this is Chris Date, for whom "brief introduction" actually amounts to 15 pages of closely-typed prose including footnotes and endnotes!

Follow the above link to the Google Books copy of the book, which should open in your browser, where the chapter seem to be available in full. After you've read the chapter, I'm sure you shall have a good understanding of the concept of logical difference.