With System 6 through Mac OS 9, the Finder searches its desktop database for an application that can open files with the same file type as the document. The Finder searches for the application in this order:
1. An application that is already open, having the same creator code as the file.
2. An application on the same volume as the document being opened.
3. An application on the startup volume.
4. An application on any available mounted volume.
In the event that multiple versions of the same application are found on one volume, the Finder opens the application whose record in the desktop database was updated last. This is usually the last version installed.
For related information, refer to the following documents:
Document 55381: "Mac OS: File Type and Creator Codes, and File Formats"
Document 10182: "Mac OS: Rebuilding Desktop File and Icon Recovery"