While DangerOS uses .jar files for applications, they are uploaded to the device in special files called "bundles". Bundle files have the extension '.bndl'. Each bundle file is linked to a specific operating system version and build number. For example, a bundle file for v3.4/155053 (T-Mobile Sidekick 3) would be denied installation on a v3.3/149695 device (T-Mobile Sidekick iD). Installation of bundles require a developer key to be installed on your device if you are using a Production OS. Internal OS builds do not require developer keys.