in "18.6. REALM OPERATIONS, Listing existing realms" are switched commands (the second sentence is about comma separated values, but the command with comma separated values has already been used.):
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Often that is too much information as we may only be interested in realm name, or - for example - if it is enabled or not. You can specify the attributes to return by using --fields option:

$ kcadm.sh get realms --fields realm,enabled

You may even display the result as comma separated values:

$ kcadm.sh get realms --fields realm --format csv --noquotes