The iPhone XS and XS Max, which ship this week, as well as the forthcoming iPhone XR support certain NFC card transactions even when iOS is not running and the host device is powered because it needs to be charged.

Buried in Apple's released technical specs for the new iPhone XS and XR lines, due to arrive Sept. 21 and Oct. 26, respectively, is a first for the devices: Express Card with power reserve. The feature allows new iPhones to conduct supported NFC transactions without the assistance of iOS.

The specs for the iPhone XS list "Express Cards with power reserve" under all models. Apple's recently published iOS security guide update provides more detail.

"If iOS isn't running because iPhone needs to be charged, there may still be enough power in the battery to support Express Card transactions. Supported iPhone devices automatically support this feature with a transit card designated as the Express Transit card [and] student ID cards with Express Mode turned on."

In addition, the security guide notes that in iOS 12, pressing the side button will show text indicating both the low battery icon and text showing that Express Cards may be used.

"The NFC controller performs express card transactions under the same conditions as when iOS is running, except that transactions are indicated with only haptic notification," the paper continues.

Notably, users are only able to access power reserve functionality under low power conditions, meaning the feature is unavailable following a standard hardware shutdown.

Apple, which originally listed the "Express Cards with power reserve" as available in a few countries, has now added that language to its spec pages worldwide. It is now known, according to a report by AtaDistance, that Apple Pay Express Transit Cards will work with power reserve in Japan and China, although it's unclear if they will work elsewhere. Apple added global FeliCa support to Apple Pay a year ago with the iPhone 8, iPhone X and the Apple Watch Series 3.