Meaning of contexts from pg_settings view

The following query, give six differents kind of context presents on the
system view pg_settings, that list all postgresql current parameters
setting.

postgres=# select distinct context from pg_settings;
  context
------------
 backend
 internal
 postmaster
 sighup
 superuser
 user
(6 rows)

What's the mean for 'backend', 'postmaster' and 'sighup' context of a
parameter? How i can know if a parameter is modifiable only with a
server restart, rather than a 'pg_ctl reload' (sighup?) or a "SET/ALTER
USER/ALTER DATABASE" command?

Best regards,
Francesco.

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Francesco Dalla Ca'
Settore Gestione Sistemi CINECA
Via Magnanelli 6/3  40033 Casalecchio di Reno (BO)
Bologna, Italy             http://www.cineca.it
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