Electromagnetic Radiation's Spectral Energy Distribution (SED) is a function, i.e., plot of brightness or Flux density versus the Frequency or Wavelength. When measured on Earth, the effects of Earth Atmosphere are relevant. A "complete" distribution often requires multiple instruments, leading to challenges in calibrating the data. A distribution over a limited Bandwidth often provides signatures for specific types sources.
SED Fitting (aka Spectral Energy Distribution Fitting) is a method of classifying distant galaxies, to help determine their age and degree of Star Formation. A challenge is that a young galaxy with Dust has a SED across Ultraviolet, Visible Light, and Near Infrared as an old galaxy. A fitting method is Markov Chain Monte Carlo (MCMC), using Random Walks around a parameter-space to determine the degree of unique fit between an SED and a known type of object. GalMC is an example of software that takes this approach.