CS Extension is part of the Collaborative Stage Data Collection System (CS), and was implemented in 2004. It is used to derive some AJCC T-values and some SEER Summary Stage codes.
Some detective work is required to interpret codes in the CS Extension field. The codes differ by type of cancer and by the version of CS in which the case was coded. In the PUF, CS fields are retained in the form in which they were submitted. That means that it will be necessary to identify the CS Version Numbers that are used in the PUF file, and use those to identify whether the contents of the CS Extension field may have changed over time. Links to the site-specific codes can be found at http://ncdbpuf.facs.org/?q=node/370.