According to people familiar with Apple's OS X Mavericks 10.9.3 beta seed 13D17, the company is requesting testers focus on graphics drivers and audio. These are the same focus areas were when the first beta rolled out last week.
It was later discovered that the first pre-release version of OS X 10.9.3 enabled a so-called "Retina" output mode for those connecting late-2013 MacBook Pro with Retina display models to external 4K screens. The feature is actually a new pixel-doubling scaling option that can display a desktop at native "Retina" levels over HDMI.
The current OS X 10.9.2 released in late February does not support pixel-doubling, leaving Retina MacBook Pro owners limited to a max scaled output resolution of 4,096 pixels-by-2,160 pixels at 24Hz.
With the new scaling options, Retina MacBook Pros can also output to a 4K display at 60Hz. Currently, the refresh rate is capped at 30Hz, making animations choppy and nearly unusable.
Developers can download the latest OS X 10.9.3 beta version via Apple's Developer Center.