Rasters store image data. Rasters may be shared between textures, including across features. Raster information is not stored directly on textures, but in a central location called the FMELibrary. Textures only store a unique identifier to rasters, called raster references. References to rasters shared in the FMELibrary are integers greater than 0. Each texture has, at most, one raster reference.
For more information about rasters, including their properties, see the Rasters description in the FME Geometry Model.