Macintosh type/creator code
A Macintosh type code is a file format identifier primarily associated with classic Mac OS. It is in the form of a FourCC.
A creator code is similar, but identifies the application that created the file, and/or the application that should be used to open it. Often, the combination of both the creator code and type code is needed to denote, or to clarify, a file's format.
These codes are essentially the classic Mac OS version of a file extension. They are part of a file's standard metadata in Macintosh filesystems such as HFS. They are found in many Mac-specific archiving formats, often via Finder information.