Apple’s in-app auto-renewable subscription system is restricted to apps that deliver new content during each renewal period – such as magazines issues – rather than ongoing services, according to the founder of bookmarking app Instapaper.

Although Apple’s in-app purchase system is capable of auto-renewable and non-renewing subscriptions, it appears to only allow the auto-renewable functionality for media apps.

However, this isn’t made clear by Apple during the submission process, according to Instapaper founder Marco Arment, who blogged that he had a submission for Instapaper 4.0, including auto-renewable subscription functionality, rejected by Apple.

The app was declined despite Apple’s documentation suggesting non-renewing subscriptions are “an older mechanism for creating products with a limited duration” and that developers should “consider using auto-renewable subscriptions instead.”

Arment said his submission appeared to violate “an unwritten rule” that auto-renewal can only be used for apps that deliver new content during each renewal period.

The developer had to ship Instapaper with non-renewing subscription, but feels that Apple needs to improve what it allows developers to do. “I hope that, in time, they unbundle some of these myopically targeted enhancements and make them potentially useful to all developers,” he wrote.