CA Editor, David Weisgerber says:

"CAS Coverage of Chemical Information on the Internet

Chemical Abstracts Service (CAS) abstract and index coverage of chemical
science resources on the Internet is currently limited to electronic-only
journals and online web-based conferences.

Internet documents are selected for coverage on the basis of criteria that
are similar to those applied to the corresponding printed journals and
conference proceedings.  These criteria are:

1.  The documents report new information of chemical or chemical
    engineering interest.

2.  They present information in a form similar to that of a scientific
    paper.  This means that they present novel findings with some
    discussion of the experimental details, research, or theoretical
    considerations that support the findings.

3.  They identify authorship, either personal or organizational.

4.  They are publicly available (not necessarily free, but not restricted
    in some manner to a particular audience).

5.  They are expected to have some relative permanence or continued
    accessibility through a citable electronic address.

6.  They are original publications (i.e., they have not been published
    previously in another form or medium) and their content is expected to
    remain unchanged.

CAS coverage currently includes articles published in electronic-only
journals and presented during online web-based conferences.  It does not
include preprints, news releases, websites, abstracts, or messages placed
on electronic bulletin boards or listservs.

In the cases of established print journals that are also made available in
electronic full-text forms, the printed journals remain the primary sources
for CAS coverage.