When you select a single file, open a Get Info window, and choose a different, non-default application, this Launch Services binding is saved in the resource fork of the file itself, in a
'usro' (User Override) resource. As far as I know, the format of this resource is private, determined by Apple, and is subject to change. (The all lowercase characters in the OSType name provide a clue, since Apple reserves file type, creator codes and resource types with all lowercase characters. For example,
'icns' (Icon Suite) resource type,
'ttxt' (originally from Teach Text, but is now the creator code (CFBundleSignature) of TextEdit.app), etc.).
This resource is created using the Resource Manager functions to modify the fork of the file, which in turn updates the file's modification date.
An example file which I just tried this procedure on is shown in the image below, opened in Resorcerer. (Special note: you will notice in the left column that there are 2 types listed:
'usro'. There is in fact only a single
'usro' entry; Resorcerer is showing that there's a
'TEXT' resource entry because it notices the content in the file's data fork is text based (a source code header file), and is therefore providing convenient access to it). The second image shows the contents of this resource, which basically contains a path to the target application I chose.
'usro' User Override resource is saved as part of the file itself, that explains why the Launch Services Programming Guide: Choosing the Binding Preference for a File claims the following:
Note: Explicit binding preferences for individual items are not user-specific but systemwide—that is, they continue to apply to the given item on that same computer, even if a different user logs in.
Now, to go one step further. If you now click the
Change All button to make this a global override, Launch Services will remove the
'usro' resource entry and instead create a global binding which is stored in the com.apple.LaunchServices.plist (per-user preferences) file. Removing the
'usro' entry is done by using the Resource Manager functions, which will update the file's modification date. Note that while it does remove the
'usro' resource, it may not delete the entire resource fork. In most cases, 286 bytes will remain in the resource fork, which is basically the minimum amount of space taken up by the resource header and resource map.