Every operating system needs a mechanism to associate documents with the applications that created them. When you double-click a Microsoft Word document icon, for example, it’s clear that you want Microsoft Word to launch and open the document.
In Windows, most documents bear a three-letter file name extension. If you double-click something called memo.doc, it opens in Microsoft Word. If you double-click memo.wri, it opens in Microsoft Write, and so on.
Mac OS 9 used a similar system, except that you never saw the identifying codes. Instead, it relied on invisible, four-letter creator codes and type codes. Apple carefully monitored and tracked these four-letter codes, in conjunction with the various Mac software companies, so that no two creator codes were alike.
When devising Mac OS X, therefore, Apple had quite a challenge: it had to create a Macintosh/Unix hybrid that somehow respected creator codes (so that old Mac OS 9 documents would open), and file name suffixes (so Windows documents would open), and the complex internal database of file types that Unix itself uses.