By convention, Apple uses four character codes in place of integer enumerations in almost all of its software, protocols, and file formats.   

Notes:
 * Four character codes are literally four ../MacRoman characters (including {{{\x00}}}, but usually four 7-bit ASCII safe characters).  
 * When "packed" as an integer, they are almost always stored in big endian format as a 32bit signed integer (e.g. {{{strnAsInteger = struct.unpack(">i", 'strn')}}}).
 * You may see crazy stuff like © or ™ in four character codes (or maybe just garbage, depending on the text editor).  The text encoding used is always MacRoman.  Fortunately, the general case is the 7-bit subset of ../MacRoman that overlaps with ASCII.
 * Four character codes are supposed to be globally unique in context, but not in general.
 * Apple maintains a registry of application signatures via Developer Technical Support.  This list is not publically available.
 * Apple maintains exclusive "ownership" of all lowercase four character codes for their software and APIs.

Examples:
 * {{{'hook'}}} - the application signature for iTunes (notice that it is an Apple product and is therefore lowercase)
 * {{{'8BIM'}}} - the application signature for Adobe Photoshop.
 * {{{'shor'}}} - the [[../AEDesc]] type code for a 16bit signed integer, also known as {{{typeSInt16}}}, {{{typeSMInt}}}, or {{{typeShortInteger}}}

In Python, four character codes are passed around as four character {{{str}}} (primarily for introspection purposes).  Some Python code will automatically interchange four character codes with {{{str}}}, {{{unicode}}}, {{{int}}}, or {{{long}}} but that will not be the general case until Python 2.4 at the earliest.  [[../bgen]] may output constants that look like {{{FOUR_CHAR_CODE('shor')}}}, however {{{FOUR_CHAR_CODE}}} is (currently) a no-op and just returns the input string unchanged.

In C, four character codes are interchangable with 32 bit integer types and will usually be defined in an enumerator:
{{{
/*  
    This is how you will see them in Universal Interfaces format 
*/
enum {
    MyConstant = FOUR_CHAR_CODE('MooV'),
    MyOtherConstant = FOUR_CHAR_CODE('©xxx')
}

/*  
    This is the "new way" that you will see in OS X frameworks
*/
enum {
    MyConstant = 'MooV',
    MyOtherConstant = (long)0xA9787878 /* so that the source isn't tainted with high ascii */
}
}}}

See also:
 * ../AppleEvents
 * ../AppleScript