You are right, Lothar. I need to be more precise: User variables can not be used in any marker type.
In index markers a number of building blocks are valid (such as <$startrange>). In cross reference markers building blocks such as <$paranum> or <$pagenum> are valid. Some of these are system variables such as <$chapnum> - which you mentioned in your example.
All system- and user variables are available on master pages. On body pages and reference pages all user variables are available, but not all system variables (for example <$curpagenum>).
You can find a complete table of these building blocks in
[1] S. O'Keefe and S. A. Loring, FrameMaker 7: The complete reference. New York, NY: McGraw-Hill, 2002.
[2] M. Sullivan and S. O'Keefe, Publishing fundamentals: Unstructured FrameMaker 11. Research Triangle Park, N.C: Scriptorium Press, 2013.