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.