Valve is fighting to stop Apple from demanding troves of information as part of its fight with 'Fortnite' maker Epic Games

• Court documents show Apple is trying to subpoena Valve in its fight with Epic Games.
• Valve has refused to hand over certain information about its revenues and the games it hosts.
• Valve claims Apple is asking for too much information, given it is not a mobile platform.

Apple's fight with Epic Games is apparently dragging other video game companies into its net.

A joint court letter filed on Thursday and spotted by Ars Technica reveals Apple has been trying to subpoena Valve, the company behind the hugely popular online game store Steam, for information.

Apple has been engaged in a legal battle with Epic Games, the studio behind the wildly popular "Fortnite," since August. The fight centers around Epic Games' refusal to comply with Apple's App Store rules on in-app payments in its iOS version of "Fortnite."

Apple requires app developers to use its own payment system, which levies an automatic 30% tax on in-app purchases. Epic Games claims this is anti-competitive, and in August decided to implement its own payment system, after which Apple kicked "Fortnite" off the App Store.

Epic Games subsequently sued Apple, and the two companies have been battling it out ever since - with Apple CEO Tim Cook scheduled to give a seven-hour deposition ahead of the case going to court in May. Epic Games has kept up the pressure, this week filing an antitrust complaint with the EU.

Valve is not directly involved in the legal fight between the two companies. Apple said it is asking for documents showing Valve's yearly sales and revenues as well as information about each app on Steam, including its pricing, as a way of gaining an understanding of the market Epic Games operates in.

Apple complains in its letter that Valve has resisted complying with some of its requests for information, and that when it has handed over information it's been heavily redacted.

Valve claims Apple is asking for too much information, given Valve is not a concerned party in its fight with Epic Games and it is not a mobile platform. More broadly, Valve is also claiming that because it's a distributor of PC games, the information it can provide is largely irrelevant to the fight between Apple and Epic Games.

"Valve does not make or sell phones, tablets, or video games for mobile devices, or otherwise compete in the mobile market," it says in the letter. It added that Apple's requests would impose an "extraordinary burden" on Valve to collect all the data Apple wants.

"The extensive and highly confidential information Apple demands about a subset of the PC games available on Steam does not show the size or parameters of the relevant market and would be massively burdensome to pull together," Valve said in its letter.

Apple argues that Valve should be compelled to offer the information because Samsung complied with similar requests. Valve's counter-argument is that Samsung is a public company so it is used to keeping records of that kind of information, which could be produced much more quickly and easily.

"Somehow, in a dispute over mobile apps, a maker of PC games that does not compete in the mobile market or sell 'apps' is being portrayed as a key figure. It's not," Valve said.