bpf group delay pp
Group delay is group delay, ie Δphase/Δf.
When you plot group delay on a graph, some group delay's look linear, some look parabolic (certain filters), most is some combination of the two. Usually communications systems can stand a fair amount of parabolic group delay, but can only tolerate less linear group delay. In other words, the shap of the graphed group delay matters.
Lets say you measure some rf component and its group delay looks mostly "linear" with a negative slope. That means the higher frequency energy gets much more phase shift per Δf than the lower frequency energy. If it is a phase modulated signal, think how that would screw up the modulation constellation. Higher frequency components rotate more, while lower frequency components rotate less. If it was instead a flat group delay (like an ideal transmission line), all frequency components rotate by the same amount, and the decision constellation is unchanged. You could only tolerate so much group delay slope before you started to make bit errors.