Elastic promises "open"—delivers proprietary

> Derived work (or more commonly "derivative work") is a term of art in copyright law. Judges decide what it is; licenses don't.

I presume that's the correct legal stance. But in my experience, when legal theories clash with reality, it's the legal theories that bend.

The reality in this case is some very prominent open source projects define what they consider a derivative work to be, and it would be a very game lawyer who took that on. For example, the prime distinction between the GPL / LGPL / AGPL is their definition of what a derivative work is. The kernel has also drawn their own line in the sand on the subject.

Maybe your answer is "they can weaken the definition of derivative work (ie, define less things to be derived), but not strengthen it beyond what it means in law". If so "Judges decide what it is; licenses don't" isn't the clearest way of saying that. Licenses have been defining it for years now, and from what I can tell doing so very successfully.

The interesting thing for me is how they draw this line. They don't use legal terms. They use terms that to us are clear and well defined, "linking", "header files", "kernel API" and so on, thus providing certainty about what is covered and what isn't. It proves you don't need difficult to reason about / need to be decided by a Judge definitions to achieve useful outcomes.

This clause looks to me to fall squarely in the "difficult to reason about / need to be decided by a Judge" definition:

> offering a service the value of which entirely or primarily derives from the value of the Program or modified version, or offering a service that accomplishes for users the primary purpose of the Program or modified version

Lawyers can have a field day defining what "derives value from" means. Does a backup program derive value from the data it is backing up? Does the kernel derive it's value from the applications it runs?