On the company's Developer Blog, Microsoft program manager Daniel Schlegel talks about the new Variable refresh rate (VRR) support in Windows 10 May 2019 Update (version 1903). Users with supporting hardware will now have access to a feature that will enable VRR support for DirectX 11 full-screen games that do not support VRR natively.
This new Windows 10 feature is intended to augment NVIDIA G-SYNC and AMD FreeSync:
This new OS support is only to augment these experiences and does not replace them. You should continue to use G-SYNC / Adaptive-Sync normally. This toggle doesn’t override any of the settings you’ve already configured in the G-SYNC or Adaptive-Sync control panels.
This new toggle enables VRR support for DX11 full-screen games that did not support VRR natively, so these games can now benefit from your VRR hardware.
The VRR option is exclusively available on PCs with a GPU with WDDM 2.6 or above drivers, that supports G-SYNC / Adaptive-Sync and this new OS feature, and a display with Adaptive-Sync or G-SYNC support.