Process 56 is the equivalent of 51, with the bottom quarks replaced by charm. Although the charm mass is neglected, the calculation contains diagrams with two gluons in the initial state and a Z coupling to the heavy quark line – hence the dependence upon the quark flavour.
When removebr is true in process 56, the Z boson does not decay.
nplotter_Zbbbar.f is the default plotting routine.