Understanding certificate revocation configuration (crl.cnf/openssl.cnf) file

I have a crl.cnf as below:

[ ca ]
default_ca      = CA_default            # The default ca section

####################################################################
[ CA_default ]

dir             = _CA_dir_
database        = _index_fname_ # database index file.

# Comment out the following two lines for the "traditional"
# (and highly broken) format.
name_opt        = ca_default            # Subject Name options
cert_opt        = ca_default            # Certificate field options

In the above file, What does CA_dir , index_fname and ca_default variables refer to or how to find what values those variables hold? It would be really helpful if someone helps me to understand this. Thanks in advance!

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