Downloadable games must ship on Xbox Live Arcade at least simultaneously with competing platforms or Microsoft may refuse to publish them, Eurogamer can reveal.
Microsoft also demands publishers release games with at least the same content "on-disc" across platforms.
Speaking to Eurogamer in a new interview, European Xbox boss Chris Lewis confirmed and defended the policy.
"We're a little biased, so obviously we're going to look to protect our own space as best we can and get exclusivity," he said.
"Whilst I can't be specific about the terms and conditions, you can be very confident we seek to maximise our own advantage to ensure the playing field is even, and certainly plays to our advantage wherever possible.
"As you can also imagine, our partners have to be mindful of the relationship they have with all platform holders, and they need to be equitable. But there are contractual situations where we get agreement with different people to do different things, and through what we have available on Xbox Live, we are able to offer things other people can't offer, that allows that exclusivity and unique elements to it that might not otherwise be available elsewhere."
Microsoft's Content Submission and Release Policy, seen by Eurogamer, details its third-party publisher guidelines.
"Titles for Xbox 360 must ship at least simultaneously with other video game platform, and must have at least feature and content parity on-disc with the other video game platform versions in all regions where the title is available," it reads.
"If these conditions are not met, Microsoft reserves the right to not allow the content to be released on Xbox 360."
This also applies to Xbox Live Arcade games. Other Online Content "must simultaneously release on Xbox Live Marketplace in all regions where the game is available". Any demo on Xbox Live Marketplace "must ship within the same week of its launch on other video game platforms or via magazines".
Sony's policy, however, appears to be more relaxed. Generally, games that launch on Xbox Live Arcade first eventually emerge on PSN. Limbo, Castlevania: Harmony of Despair and Lara Croft: Guardian of Light are just a few high-profile examples.
While Lewis refused to rule out the possibility of publishing downloadable games that launch on PSN first in the future, "I'd be surprised if we saw that as something we'd encourage," he said.
"But, honestly, and this is going to sound a bit contrived, we just want what our consumers want from us. We want to be where they want us to be. We want the quality bar of what they experience from us to continue to go up. I think it has to happen. Everybody's got to do that. If we want to continue to command healthy average selling prices, which we all do, that which we offer our consumers has got to keep getting better.
"Despite the fact it can be irksome to have such strong competition all the time, it actually does keep us on our toes. It's great for everyone, and it makes for a very healthy race to higher and higher levels of quality of game experiences."
Not all agree, however. One representative from a publisher who wished to remain anonymous told Eurogamer Microsoft's policy blocks developers from taking advantage of other platforms' strengths.
"Microsoft is suggesting that anything but parity will result in them not carrying a title. They may think this is competitive, but it's not. They are killing any creative exposure of titles to make up for their own platform's shortcomings."
Last year Eurogamer revealed that Summer of Arcade games must remain Xbox Live Arcade exclusive for at least four weeks.