Apple is facing the US Supreme Court to defend the commission it makes on iPhone app sales.
The company takes a 30% commission on every sale and is accused by a group of consumers of breaching anti-trust laws because there is no alternative place to buy an iPhone app.
Apple says it does not own or sell the apps.
The court must decide whether there is a case if there is no direct link between Apple and the app buyers.
While app developers set their own prices, Apple collects the payments.
Apple is appealing against a previous decision made by a lower court. The case began in 2011 and was revived last year.
Apple says if the legal action is allowed to proceed, it could affect other technology giants as well - Google and Microsoft, for example, also take 30% commission on app sales.
Jack Kent, an analyst at IHS Markit, told the BBC Apple's App Store was an increasingly important part of its business.
"The Services category - which includes App Store commissions, alongside other media, content and services - is now Apple's second biggest driver of revenue after iPhone sales and has been its fastest growing source of revenues," he said.
"As many of Apple's device categories mature, services are vitally important as a means of driving incremental revenue after the device sale and also for tying users in to Apple's ecosystem of devices, apps and services."