According to a source, the build includes final release notes for the update, highlighting fixes for Mail, AFP file transfer performance, screen savers, and Xsan.
-Fixes an issue that may prevent Mail from displaying messagesWhile a prerelease build for Apple employees is frequently indicative of an imminent public launch, it is not always the case. As was seen with OS X 10.8.3 back in early March, Apple issued a prerelease build to employees but then elected to distribute a new developer build roughly a week later with a public release following two days later.
-Improves AFP file transfer performance over 802.11ac Wi-Fi
-Resolves an issue that may prevent a screen saver from starting automatically
-Improves Xsan reliability