It is explained more precisely in specific guidelines for developers. Some tasks may be split more efficiently than others. That way, applications keep using standard API (OpenGL or DirectX) and the driver splits tasks between two physical video cards. The main purpose of Crossfire (by ATI/AMD) and SLI (by NVIDIA) technologies is to unify a pair of 3D accelerators for increased performance. NOTE: Starting from Parallels Desktop 15, 3D acceleration is enabled by default in Windows virtual machines. A special checkbox in configuration of Windows virtual machines allowed you to disable or change the level of 3D acceleration in Windows has been removed due to very low usage of the feature. To make sure this option is enabled go to virtual machine configuration window > Hardware > Graphics > Advanced Settings Note: Parallels Desktop does not control when to switch between graphics cards. To learn more about the feature, please visit the Apple's KB HT202043. If your Mac has discrete graphics, Parallels Desktop can use the macOS 'Automatic graphics switching' for its virtual machine(s). It provides a more economical alternative to the standalone card, known as 'discrete graphics' or 'dedicated graphics.' In this case, Parallels Desktop will use the resources of the built-in Mac built-in graphics. Most Mac computers have an integrated graphics card, which is built into the motherboard and shares memory with the CPU. The actual acceleration is achieved by translating Direct X commands from the guest OS to the OpenGL API on the macOS side.

Instead, Parallels Display Adapter driver (which is part of the Parallels Tools installation) interfaces with virtual hardware and provides 3D acceleration features.

Parallels Desktop has no access to the Mac's device's physical graphics cards.

How to update mac graphics card drivers