Back to MU: MU has J, C, D, I as business buckers (with Q recently added), SU has J, C, D, I, Z.

Now on MU operated by SU the availability is 'free sale', however, MU made the following mapping:

MU SU
C J
D C
I D

While MU I fare level is that of SU Z fare...Moreover, even lowest SU Z fares have higher revenue per mile than MU I fares. Who know how they do the math further