Used by the message catalog functions catopen, catgets and catclose to identify a catalog.
nl_item
Used by nl_langinfo to identify items of langinfo data. Values for objects of type nl_item are defined in <langinfo.h>.
NL_SETD
Used by gencat when no $set directive is specified in a message text source file. This constant can be used in subsequent calls to catgets as the value
of the set identifier parameter.