/etc/postgresql/postgresql.conf is used to store settings which should
always apply to a database connection, at least when it starts.

The possible entries in this file are described in section 3.4 of the
PostgreSQL Administartor's Guide (package postgresql-doc).  These describe
settings which can be done in the file or on the command line of the postmaster
or by the SQL command SET (not all can be done by all of those).  

If an unknown option is found in this file, the postmaster will abort.