Most of the time, an external link is easy to recognize because it starts with http:// (or https://). However, when the external link is ambiguous (i.e., it could be a tiddler title), then the 'ext' keyword is needed to ensure that an external link is created.
For example, suppose I have a tiddler named "foo/bar/mumble.txt"... then [[click me|foo/bar/mumble.txt]] would be an internal tiddler link. Now suppose that there is also a local file named "foo/bar/mumble.txt" that is stored in a subdirectory of the current working folder. To create an external link to this file, you can't use the "http://" prefix, as that would make the reference absolute (i.e., no longer relative to the working directory). Adding the 'ext' keyword forces the link to be external, even if there is a tiddler with the same name.
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-e