Graphics API items in Xbox One are mainly DirectX APIs. Graphics APIs also include DDS Texture Loader and DirectXTex, which are libraries that are used by the samples.
The Reference documentation here is customized for Xbox One. This documentation covers the API items in the Xbox One version of the .h files. For Programmers Guide content, links to MSDN are provided, showing general platform-agnostic coverage.
DirectX API Reference
API Reference for the Xbox One version of DirectX 11.1 and DirectX 12, including Xbox One-specific APIs. DirectX graphics provides a set of APIs that you can use to create games and other high-performance multimedia apps. DirectX graphics includes support for high-performance 2-D and 3-D graphics.
DDSTextureLoader Module
Lightweight module supporting the loading of Direct Draw Surface (DDS) files as textures.
DirectXTex Library
The DirectXTex library supports many texture file formats. Its primary use is in creating texture tools.