The caldb_version.fits file stores which version of the CALDB you are running on your local system. This file is automatically updated each time the CALDB is upgraded on your system, so the final row indicates the current version:

unix% dmlist \
      $CALDB/docs/chandra/caldb_version/caldb_version.fits"[cols caldb_ver]" \
      data,clean | tail -1 
 4.1.1

In this case, CALDB 4.1.1 is being run. The most recent version of the CALDB may be obtained from the CIAO download page.