ATmega16U2 replaced 8U2, but it does not mean that the 16k flash of R3 can make your code run faster
The stitching just shows extension board the current on-board voltage. The other one is backup placeholder pins
The RESET circuit is more stable. RESET button position is changed, and moved to near the USB interface of voids, which is more convenient
Stitching changes: beside the AREF sutural are two extra I2C pins (SDA and SCL which are copy of Analog 4 and 5, not additional I2C interface)
Beside the RESET also are two extra pins, one is IOREF which supports the extension board stand load voltage

Technical Details

Size (LWH): 5.4 inches, 4 inches, 0.5 inches
Product Details
Product Dimensions: 5.4 x 4 x 0.5 inches
Shipping Weight: 0.1 pounds (View shipping rates and policies)
ASIN : B00XMPCYDG