Two of the world's biggest software companies face off in court this week for the second time, even though the most important issue of their dispute has already been resolved.

The high-profile trial that begins Monday will again include celebrity CEOs on the stand, dense expert testimony, and an utterly unpredictable outcome decided by a jury. But what's truly at stake in Oracle v. Google, round two?

For those who work with code for a living, a lot. The case revolves around how Application Programming Interfaces, or APIs, can and cannot be used. Boiled down, APIs define how different types of code communicate to each other. If owners of those APIs can use copyright law to control how programming is done, there will be a sea change in industry practices. For many developers, especially of open source software, this will be a change for the worse.

"That is really going to create a radical shift in how software is developed worldwide," Mitch Stoltz, an attorney for the Electronic Frontier Foundation who's been following the case, said in an interview. "If it requires permission each time APIs are used and code calls other code, then you've upended the economics of software."

Oracle acquired the Java language when it bought Sun Microsystems in 2009. While the Java language itself is free to use, Oracle sued Google in 2010, saying that because Google used 37 Java APIs in building Android, it had infringed Java copyrights.

The first trial ended with a clean sweep for Google when US District Judge William Alsup found that APIs shouldn't be eligible for copyright at all. But Alsup's ruling was roundly overturned by the US Court of Appeals for the Federal Circuit, which said APIs are creative works that deserve copyright protection.

Google petitioned the Supreme Court to hear its case, joined by allies including Yahoo, HP, Red Hat, and open source advocates, but was rebuffed. Microsoft, EMC, and NetApp urged the high court to let the Federal Circuit ruling stand, and that's what happened.

That didn't end the dispute, though. Even though the Federal Circuit said in no uncertain terms that APIs are copyrighted, it didn't weigh in on whether Google's use of the 37 Java APIs was fair use.

So now, it's back to a jury. Oracle has won its bid to be able to use copyright as a powerful legal sword. But Google can still dodge that sword by convincing a jury that Android's use of APIs constitutes fair use—in other words, relatively small and justified.

Oracle's $9.3 billion damage demand, not to mention significant risk for other software developers, hangs in the balance.

For developers, a second blow?

The Federal Circuit's rule allows a company to use copyright law in order to stop others from calling on its APIs and interacting with its code. In the eyes of many tech companies, software developers, and open source advocates, that's going to create a lot of unnecessary legal risk. In the wake of Oracle v. Google, calling on someone else's API could get you sued.

In the EFF's view, the Federal Circuit decision was wrong and conflicts with existing 9th Circuit cases, like Sega v. Accolade (1992) and Sony v. Connectix (2000), which allow for interoperability between systems, whether a copyright owner likes it or not.

Because of the Federal Circuit decision, a developer or company can be subject to a copyright suit over use of an API. Some of those lawsuits will be brought by copyright owners who just want to shut down competition, Stoltz said. That means developers will "spend more time talking to their lawyers, and they'll be more worried."

In a sense, then, the damage is already done. But the upcoming trial is still relevant. If Google can't win on fair use, it would be a second blow to the old notion that code should be re-usable. The enormous damages figure can't be ignored, either. Oracle has suggested it should get $8.8 billion worth of profits from Android, as well as another $475 million in actual damages. That's not chump change, even to Google. For smaller companies, a big Oracle win will make it clear that if they lose a lawsuit over unauthorized use of APIs, they could suffer an extinction-level event.

In an odd twist, the only reason the Federal Circuit heard the case at all is because the original lawsuit included a patent claim, and the Federal Circuit hears all patent appeals. The DC-based court kept jurisdiction even though Google prevailed on the patent claims, and Oracle didn't bother to appeal that part of the suit. If the patent claim hadn't been there to begin with, the case would have gone to the San Francisco-based 9th US Circuit Court of Appeals, where Google would likely have won.

The current landscape—with the Federal Circuit sympathetic to API copyright claims while other circuits are much more skeptical—could result in copyright owners throwing in a patent claim, just to make sure their case goes to the Federal Circuit.

Unpredictable situation

It's a complicated mix of elements to send to a jury, with unpredictable results. Even for judges, applying the four-factor test of fair use isn't simple, and the weight of the various factors can change from case to case. Attorneys for Oracle and Google have engaged in heated pre-trial battles over exactly how fair use should be explained to the jury.

The set of instructions, published by Judge Alsup on May 3, informs the jurors that "Google did not have the right to use the exact lines of declaring code and the overall structure, sequence, and organization of the 37 API packages," but also instructs them to consider "the extent to which you find it was necessary" for Google to use the declaring code and organization of the 37 API packages to write in the Java language.

Ultimately, it will come down to which of two competing narratives the jury finds more plausible, suggests Stoltz.

On the one hand, Oracle lawyers will portray Google as seeking something for nothing by not doing the hard work of building its own APIs from scratch—which Google could have done.

Google will push back, emphasizing that the Java language itself is free to use. By building Android, Google did something new and useful with Java—exactly the kind of creation that copyright law is supposed to encourage.

Jury selection is scheduled for Monday in San Francisco federal court.