What types of plans can we snapshot?

I am on the RC3 bits and I am not able to snapshot all plan types and I have tried a number of different combination and am still not clear for most options the snapshot is disabled for me until I am selecting certain iterations.

The doc: http://cclbld40.beaverton.ibm.com:9082/CLM30help/index.jsp?topic=/com.ibm.team.apt.doc/topics/c_snapshots.html

Talks about the types, and where the Proposed type can be used, but my question is where can I find what types of plans can be snapshot-ed ? If Regular and Planned snapshots should be available for any plan, I will file a defect.

-Sean