public sealed class Texture2DArray : Texture

Modern graphics APIs (e.g. D3D10 and later, OpenGL ES 3.0 and later, Metal etc.) support "texture arrays", which is an array of same size & format textures. From the shader side, they are treated as a single resource, and sampling them needs an extra coordinate that indicates which array element to sample from.

Typically texture arrays are useful as an alternative for texture atlases, or in other cases where objects use a set of same-sized textures (e.g. terrains).

Currently in Unity texture arrays do not have an import pipeline for them, and must be created from code, either at runtime or in editor scripts. Using Graphics.CopyTexture is useful for fast copying of pixel data from regular 2D textures into elements of a texture array. From editor scripts, a common way of creating serialized texture array is to create it, fill with data (either via Graphics.CopyTexture from regular 2D textures, or via Texture2DArray.SetPixels or Texture2DArray.SetPixels32) and save it as an asset via AssetDatabase.CreateAsset.

Note that not all platforms and GPUs support texture arrays; for example Direct3D9 does not. Use SystemInfo.supports2DArrayTextures to check. Also, this class does not support Texture2DArray creation with a Crunch compression TextureFormat.