Solved Check hashes on base system files

I know we can check the checksums on installed packages with pkg check -s -a
Is there any way to do the same thing with the files of the base system?
IDS       Compare the system against a "known good" index of the
          installed release.
What does it mean - "against a known good index"? Where does it get such index in case of XX.X-RELEASE-pN where N>0?