User's Guide - Key Concepts
Some objects allow you to specify an external reference to the file you want the object to use. An external reference is just a path that points to where the file will be at run time. In other words, you're specifying where the file will already exist when the user runs your AutoPlay application. It's like saying "don't worry about this file for now; just wait until run time, and then look for it here."
Note that when this option is enabled, AutoPlay doesn't do anything to get the file where it needs to be at run time—instead, that's left entirely up to you.
For objects that support them, you can enable the external reference option by selecting the External reference checkbox located below the field where the path is displayed.
The External reference checkbox
Files that AutoPlay accesses as external references are known as external files.
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