dojo.global is an alias for the global scope.
In a browser environment, dojo.global is the window object. You should refer to dojo.global rather than window so that your code will run correctly in other contexts (e.g. Rhino on a server).
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Though this example is clearly targeted at a browser environment, by using dojo.global over the window object we are ensuring the code will run in any other environments with a defined host environment file (hostenv_something).