The Texture3D type is an asset type that represents the the usage, reference, and the data of a 3D file. This file is usually represented as a .cube file, and can have many layers of images that make up this 3D texture.
Exporting it using the VolumeExportable will make this file for you, and bringing it back into Resonite will automatically pre-setup it so it can be usable within Resonite, having the materials, textures, slots, and even becoming exportable again.