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  • Chris Kang thinks this is interesting:

In the previous example, the main installation directory is referenced by placing the Id attribute of its corresponding Directory element in square brackets, which is then followed by the name of the file. Even if the path of MyProgramDir changes, it will still lead us to the InstallMe.txt file.

From

Cover of WiX 3.6: A Developer's Guide to Windows Installer XML

Note

[xxx] will always point xxx to a previously defined Id value. Whatever that Id represents will be placed where [xxx] is. If the entity represented by the Id is a string, it will be that string value.