Creator code and type code are file attributes to detemined an application to open the file, which are traditionally used through Mac OS 9. Each codes are expressed with four ascii characters. The meaning of each attributes are as follows.
In Mac OS X, the file name extension are widley used to determine the application to open the file. But in many cases, only file extension can not give enough flexibility.
For example, you may want to open some file of which extension is ".pdf" with "Preview.app" and others with "Adobe Reader". When the file have a creator code, the application to open the file is determined by the creator code. Therefore you can choose the application to open the file by setting a creator code to the file.
You can bind an application to a file to open it in the infomation window of Finder. But an ungly mechanism is used to do this. A full path of the application is recorded in the resource of the file. This means the binding between the file and application is easily lost when the location of the application is moved. Also custom icon is pasted on the file. Therefor you can not know the actual application to open it from the looks of the file in some cases.
QuickFileType allows flexible and certain binding of files to application using the traditional Macintosh technology until Mac OS X 10.5. In Mac OS X 10.6 or later, creator code will be ignored. The biding between files and applications are determined by default applications responding its path extensions or usro resources.