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in a short explanation you would use radiance if you only doing landuse landcover classification using hard classification (i.e nearest neighbour, paralelepiped, maximum likelihood classification) you would use reflectance if you want to extract some biophysical data from imagery spectral information (i.e vegetation water content, fPAI, percentage vegetation cover, soil moisture etc), or Landuse/landcover soft classification (i.e spectral unmixing, spectral angle mapper, artificial neural network, etc)

from above article, I quote for you :