Data documentation doesn't provide any information on interpreting this index (or where the data comes from). Other CA indices describe 100 as being the U.S. average but it's unclear whether this holds for the Crime Indices.
The crime data come from AGS. You can read more about their creation in this PDF Document.
From that document it appears as though an index of 100 is on par with national totals... "The final crime rate estimates were then weighted by population and aggregated to the national totals. The results were then scaled to match the 2010 preliminary estimates (at a state level) and converted to indexes relative to the national total."
