Tim Cook is making a rare appearance to defend his company under oath

• Apple CEO Tim Cook is taking the stand Friday as a witness in a major lawsuit.
• "Fortnite" maker Epic Games is suing Apple, and the bench trial started this month in California.
• Epic says Apple's App Store is a monopoly. Apple says Epic broke its developer contract.

Apple CEO Tim Cook is scheduled on Friday to take the stand as a witness in defense of the company he runs.

Apple and the maker of "Fortnite" are in a court battle in California - the culmination of a yearlong spat between the two American business giants.

Epic Games filed a suit against Apple last summer after its hit game "Fortnite" was pulled from Apple's App Store.

Apple says it pulled the game because Epic violated the terms of its developer agreement when Epic implemented a payment system in the game that enabled players to circumvent Apple's App Store. Epic says that the App Store is a monopoly and that iPhones and iPads are no different from computers.

The in-person trial began earlier this month at the US District Court for the Northern District of California in Oakland. Judge Yvonne Gonzalez Rogers has overseen about three weeks of hearings.

After three weeks of testimony and questioning by lawyers for both Apple and Epic, Cook is appearing as a witness for the defense as the trial wraps up.

It's Cook's first court appearance as a witness on behalf of Apple, though he's appeared at congressional hearings alongside other tech CEOs. While congressional hearings rarely result in action, the results of this trial could have a major influence on Apple's business.

There's no way of knowing what the lawyers for Apple and Epic will ask Cook, but at the heart of the fight is a philosophical disagreement on the nature of the iPhone: Epic says it's a computer, while Apple says it's fundamentally distinct. That argument is critical because of how the App Store operates, with Apple acting as the sole arbiter of what can and cannot be published on the iPhone.

If the iPhone is a computer, then the App Store is a monopoly, Epic's lawyers say. If it isn't, and it's a distinct category of device, then Apple is protecting its users by keeping alternative digital storefronts off the iPhone, according to the company.

Multiple Apple execs have already appeared as witnesses in the trial.

Craig Federighi, an Apple senior vice president, most clearly laid out the case that we're likely to hear echoed by Cook: Apple's computers are "like a car," he said, and that means users can take it off road or wherever they'd like - for better or worse - while the iPhone and iPad operating system, iOS, needs to be usable by children and even babies.

The Mac operating system, Federighi said, is less secure than iOS because of its open nature. If you were able to download iPhone and iPad apps outside Apple's App Store, Federighi said, you'd be open to a variety of security issues that Apple couldn't review before use.

This is because of the App Store's review process, he said, which offers a standard of safety across all iPhone and iPad apps.

The trial is expected to wrap up on Monday.

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