This symbol is used to indicate the reflection at the core-mantle boundary for waves incident from the mantle. For this and other such symbols used in seismic phase names see IS 2.1 in this Manual and Storchak et al. (2003 and 2011)
Peter Bormann, Keiiti Aki, William H. K. Lee, and Johannes Schweitzer (Compilers and Amenders), Glossary of Terms Interest to Earthquake and Engineering Seismologists (Version 1), Publication records from external use of GFZ scientific infrastructure, Germany 2011. http://gfzpublic.gfz-potsdam.de/pubman/item/escido c:56137:3/component/escidoc:56143/Glossary_rev1.pd f, (ND/ Erthq - Amer) {980}