Selecting this check box enables the 64-bit Double Precision Engine in SONAR throughout the entire signal path, including dithering and plug-ins.

The 64-bit Double Precision Engine provides greater resolution, meaning more accurate audio reproduction and more headroom. You'll especially notice the benefits of the 64-bit Double Precision Engine when working in large projects containing many audio tracks and plugs-ins. Your chances of clipping will be significantly reduced. 

The 64-bit signal path includes plug-ins and buses. SONAR sends and receives 64-bit data to and from all plug-ins that accept 64-bit data. If a plug-in requires 32-bit data, SONAR will automatically handle the conversion.