This is the amount of memory used by the GPU via DirectX/Direct3D. It doesn't cover memory usage via other interfaces (like OpenCL for example).
Unfortunately, AMD doesn't seem to offer an universal way how to report exact memory usage of the GPU (like nVidia), so this is currently the only way how to get a picture of memory usage.
Theoretically they should be close.
"GPU Utilization" is read straight from the GPU internal registers, so this should be the more precise value and it should cover the entire GPU.
"D3D Usage" is read via Direct3D, so it covers only this interface.
Unfortunately, hardware vendors don't give out information that would help us to understand such things, so sometimes we're just left to try and see...
Hey Guys my name is Mark.Well i think that D3D is used to provide three perspective design in programs where performance is essential, such as activities. D3D also allows programs to run fullscreen instead of included in a screen.Thanks!!