What does 'passband phase' and 'stopband phase' mean?
You seem to be overthinking this. This is a basic electronics class usage of how to do simple Bode plots.  Just like a Bode amplitude plot is not the exact amplitude plot, but a rough approximate that changes slope only at the calculated 3dB frequency points, the Bode phase plot is similar. So stopband and passband phase are both single numbers that approximate the phase before and after the 3dB points. See Wikipedia article on Bode plots for an example diagram of such Bode phase plot.