When you're shooting in RAW format, the 14-bit digital signals that are converted from analog in the image sensor are passed to the front-end LSI and BIONZ X, where they undergo 16-bit processing before converting back to 14-bit RAW image output. This leads to smoother, more natural tonal gradation in these images.
[1] Image Sensor [2] The latest high-speed front-end LSI [3] 16-bit Image processing