For the purposes of determining whether a particular activity is using Open Source Software, it's relatively simple. You simply ask whether the user or, in the case of a transfer, the recipient gets the FSF's "four freedoms". Since the patent law applies to each use and doesn't have to be carried along the chain of sublicensing and distribution, a lot of the complexity of particular licenses doesn't matter. A lot of the possibilities for licenses which are "Free" for the purposes of users but which let the owner of the original code take modifications proprietary would be protected by this clause, but that wouldn't matter too much if a proprietary version would then lose the protection.
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