[CompressedTexture3D] is the VRAM-compressed counterpart of [ImageTexture3D]. The file extension for [CompressedTexture3D] files is [code].ctex3d[/code]. This file format is internal to Godot; it is created by importing other image formats with the import system.
[CompressedTexture3D] uses VRAM compression, which allows to reduce memory usage on the GPU when rendering the texture. This also improves loading times, as VRAM-compressed textures are faster to load compared to textures using lossless compression. VRAM compression can exhibit noticeable artifacts and is intended to be used for 3D rendering, not 2D.
See [Texture3D] for a general description of 3D textures.