Answer: The tns_admin parameter determines the location of the tns administration files (e.g. tnsnames.ora and listener.ora, etc.)
The tns_admin parameter is super useful on servers with many databases on different release levels (hence different $ORACLE_HOME directories) because all databases tns_admin values can be set to a single location, allowing a single set of SQL*Net files for all databases on the server.
Without the tns_admin variable, each instance would have it's own set of TNS files in the $ORACLE_HOME/network/admin directory.
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