Problem with cross-section (NuFissionMatrix_XS)

Dear All,
I’m curious about the defination about NuFissionMatrix_XS. Why this cross-section is given as matrix and not a scalar at certain energy group like fission_XS?
Best regard

You can find the definition here. Each element in the matrix tells you the production of neutrons with energies in group g’ originating from neutrons with energies in group g.