Equation View of Heat Transfer

when the Physics "Heat Transfer" used, we can see such Variables in the Equation View:
ht.kxx Thermal conductivity, xx component
ht.kyx Thermal conductivity, yx component
ht.kzx Thermal conductivity, zx component
ht.kxy Thermal conductivity, xy component
ht.kyy Thermal conductivity, yy component
ht.kzy Thermal conductivity, zy component
ht.kxz Thermal conductivity, xz component
ht.kyz Thermal conductivity, yz component
ht.kzz Thermal conductivity, zz component
My question is why there are so many components? Why not just three components: x component, y component, and z component?