The headline feature in this build is quite a big one. Microsoft have changed a lot of how video and graphics  in general works. You can now determin, on an application by application basis, exactly how applications deal with graphics, including which graphcs card they use on systems with more than one. Here's how Microsoft desctibe it:

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HDR Video on more Windows PCs: Many newer devices are capable of HDR video, but needed to be calibrated in the factory to enable HDR. Now, we are expanding HDR video access to more users via new functionality via Settings > Apps > Video playback. If the “Stream HDR video” toggle can be switched to “On”, your device can be calibrated for HDR video.

To try our experimental calibration tool, click the link “Change calibration settings for HDR video on my built-in display”. This allows you to change the way HDR video appears on your device, allowing you to find your preferred balance between details in dark scenes and details in bright scenes. This tool is still early stage, and we’d love any feedback you might have as we are getting this tool ready for release with RS4.

Note: By default, HDR video uses the full brightness of your screen, so it consumes a little more battery. Fear not: if you want the best of both, just check the box “Don’t increase display brightness when watching HDR video on battery” under “Battery Options” in Settings > Apps > Video playback.

New Graphics settings for Multi-GPU Systems: In this build we’re introducing a new Graphics settings page for Multi-GPU systems that allows you to manage the graphics performance preference of your apps. You may be familiar with similar graphics control panels from AMD and Nvidia, and you can continue to use those control panels. When you set an application preference in the Windows Graphics settings, that will take precedence over the other control panel settings. Find the page by going to Settings > System > Display and scrolling down to the “Advanced graphics settings” link. (In future flights, you will see this link as “Graphics settings”.)

The first step is to choose an application to configure. Choosing a “Classic app” will let you browse to an application on your system. Choosing a “Universal app” will let you choose a Store application from a list. By default, the application added to the Graphics settings page is given a “System default” preference. System default means that the system decides the best GPU for your application.

Once you’ve chosen the application, click on the application in the list and then click the “Options” button. The “Power saving” mode is a request to run the application on the most power saving GPU available. The “High performance” mode is a request to run the application on the most high performance GPU available. Generally, the power saving GPU is the integrated GPU on a system, and the high performance GPU is the discrete GPU or external GPU. If you have both a discrete GPU and an external GPU on a system, the external GPU is considered the high performance GPU.

Remove an application from the list by clicking the application and clicking the “Remove” button. Removing an application is the same as choosing “System default”.

Applications are always allowed to have the ultimate choice of which GPU to use, so you may see additional applications that do not follow the preferences you set. In that case, look for a setting within the application itself to choose a preference.

As ever, full release notes are here