From a post on the FiftyThree blog:
Today we’re thrilled to announce the FiftyThree SDK, the simplest way for app developers to integrate Pencil’s powerful creative features into their own apps. We built Pencil to make creating on the iPad simple and natural with features like blend, erase, adaptive palm rejection, and the simplest Bluetooth pairing with Kiss-to-Pair™. Now with the release of our SDK, app developers anywhere can bring the Pencil experience to new platforms for creativity whether it’s games, document editors, music, or even museum installations. [...] We’ve worked hard to create an SDK to give developers access to all the powerful expressive technologies in Pencil from touch classifiers to palm rejection to Surface Pressure (coming with iOS 8). Exciting news, and a huge opportunity for third-party developers. See also: my review of the Pencil stylus.