Microsoft is fending off criticisms of its plan to unify its products and to raise the profile of its Windows Store, and the company is now planning to take that debate into its annual gathering of developers later this month.

Xbox boss Phil Spencer responded to the ongoing criticism of the Universal Windows Platform (UWP) today by reiterating that it is not a closed system. The Microsoft executive also promised to address concerns at the company’s Build conference for developers, which kicks off March 30 in San Francisco. Spencer is responding in part to an opinion piece GamesBeat published yesterday from Epic Games founder Tim Sweeney in which he meticulously argued that Microsoft is attempting to change the definition of an open software platform. The global gaming industry is worth $99.3 billion and general PC software is worth even more, but Epic doesn’t think developers in that market should have to share their revenues with Microsoft.

Sweeney has repeatedly said that the industry must stop UWP, and it’s obvious that his company — which makes the game-development tool Unreal Engine — has a lot at stake here. Epic has made efforts to get more people to use its standalone game launcher, which enables players to launch upcoming releases like the strategy shooter Paragon as well as the Unreal software. Sweeney wants to avoid a future where it has to go through Windows to get most people to install that, in which case Epic would have to give Microsoft 30 percent of any revenue it makes through a UWP channel.

Sweeney is making a lot of noise about this subject, and it’s clear that Microsoft is hearing him.

@PNF4LYFE It's not closed and yes, //build will be a good time for us to show we are hearing the feedback. — Phil Spencer (@XboxP3) March 11, 2016

But when the Windows company does go on stage to talk UWP at Build later this month, it will probably have to address Sweeney’s very specific points. Key among those is that while Windows 10 today enables every user to install apps outside of an official Windows channel, developers still need to register with Microsoft and get their software approved with a digital signature.

“Is this open? You be the judge,” writes Sweeney. “It’s certainly a departure from the win32 precedent, in which any developer can compile a program, put it on a web site, and any user can install or run it by downloading and clicking on it.”

If you tried that today, your software would likely set off a number of ominous warnings on Windows 10, which would scare off a significant portion of customers.

And that’s the key to this debate: Microsoft and Epic are trying to win the war to define “open,” and Microsoft will have to make a compelling case at Build to prevent Sweeney’s side from gaining more momentum.