The Apple Event Manager user four-character codes (Apple event codes) to identify the data within an Apple event. Since all the Apple-defined event codes are lowercase letter and space (such as 'capp' and 'cha '), so it's encouraged to name customized event codes with a heading uppercase letter (such as 'Capp' and 'Cha '). All primary Apple event codes are defined in AppleEvents.h and AERegistry.h of AE.framework.
An Apple event is uniquely identified by its event class and event ID, each of which is an arbitrary four-character code. Apple defines event class and event ID for standard Apple events. We can define application-specific Apple events our application supports.
