Trans-Neptunian spectral types
A trans-Neptunian spectral type (or Kuiper belt spectral type) is assigned to trans-Neptunian objects (TNOs) and other icy small bodies in the Solar System based on their reflectance spectrum (and in some cases, albedo). Spectral types are useful as they provide a first-order description of the surface composition of these distant objects.
Source: Wikipedia — Trans-Neptunian spectral types (CC BY-SA 4.0)