Many of the configuration parameters have limitations on who can change them and where or when they can be set. For example, to change certain parameters, you must be a Greenplum Database superuser. Other parameters can only be set at the system-level in the postgresql.conf file or require a restart of the system for the changes to take effect.
Many configuration parameters are considered session parameters. A session parameter can be set at the system-level, the database-level, the role-level or the session-level. Most session parameters can be changed by any database user within their session, but a few may require superuser permissions.