Locale categories are manifest constants used by the localization routines to specify which portion of a program's locale information will be used. The locale refers to the locality (or Country/Region) for which certain aspects of your program can be customized. Locale-dependent areas include, for example, the formatting of dates or the display format for monetary values.
Parts of program affected
All locale-specific behavior (all categories)
Behavior of strcoll and strxfrm functions
Behavior of character-handling functions (except isdigit, isxdigit, mbstowcs, and mbtowc, which are unaffected)
Same as LC_TIME
Same as LC_ALL
Monetary formatting information returned by the localeconv function
Decimal-point character for formatted output routines (for example, printf), data conversion routines, and nonmonetary formatting information returned by localeconv function
Behavior of strftime function
See setlocale, _wsetlocale for an example.