What is the difference between SCI and SCIE?
Thomson Reuters customer support explanation on SCI and SCIE:.


  The Science Citation Index (SCI) is a sub-set of the Science Citation
  Index Expanded (SCIE), containing journals that rank competitively
  among the most highly-cited core journals in their category or
  categories. The Science Citation Index Expanded is essentially the web
  version of what used to be a database available only on
  CDRom/Diskette.
  
  When selecting the journals for the Science Citation Index we choose
  the top journals from each subject category and supplement this with
  top regional journals from each category to give broad geographic and
  multidisciplinary coverage. The evaluation of and acceptance of a
  journal for the Science Citation Index Expanded or the Science
  Citation Index is essentially the same with one major difference.
  
  This difference is in the application of citation analysis to the
  journal. While every science journal in our database is covered in the
  Science Citation Index Expanded and only those journals of relatively
  significantly higher citation impact are selected for the Science
  Citation Index. In other words, Science Citation Index covers only the
  most highly cited, highest impact journals in each category. This is
  because of the constraints of the CDROM and print media there is no
  difference in the selection process for Science Citation Index and
  Science Citation Index Expanded journals.