Tim Cook will on Friday be confronted with allegations that Apple is operating an illegal monopoly, in a high-stakes case brought by Epic Games, the maker of Fortnite, where the verdict could have far-reaching consequences for 1 billion iPhone users and thousands of developers.

Cook will be among the final witnesses in the month-long trial, scheduled to end on Monday, in which Epic argues that Apple abuses its allegedly dominant position by forcing developers to distribute applications through the App Store, where it takes a commission of 15–30 percent.

Apple expelled Fortnite from its App Store last year when Epic tried to get around the fees by giving the game’s players a zero-commission way to make in-app purchases.