Difference between the minimum rising gas volume at the bottom of the distillation column and the rising gas volume in the stripping section in the 2016 case

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Question 3 in the afternoon session of the 2016 case study: 3. A factory uses continuous distillation to separate a formaldehyde mixture. The feed contains 0.55 mole fraction of formaldehyde, with a flow rate of 315 kmol/h; the feed has a dew point temperature. A total condenser is used at the top of the tower, while reflux occurs at the bubble point. The formaldehyde concentration in the distillate is 0.96, and that in the bottom stream is 0.1. If the average relative volatility of formaldehyde under these operating conditions is 2.0, what is the minimum upward gas flow rate through the tower (in kmol/h)? Question 6 in the 2016 case study: A distillation column is used to treat a benzene-toluene mixture containing 50% benzene by mole fraction; the processing rate is 100 kmol/h. The product separated at the top of the column must contain at least 98% benzene. The upward gas flow rate in the stripping section is 120 kmol/h, the reflux ratio is R = 1.3RMIN, and the feed is at the bubble point. The equilibrium relationship is y = 1.425x. What is the output rate from the top of the column? When calculating in (kmol/h), for question 3, the minimum amount of rising gas at the bottom of the tower is (R+1)D-F; for question 6, the amount of rising gas in the stripping section is (R+1)D. I’m not a chemical engineering professional, so I would appreciate it if the experts in this forum could explain the difference between these two values. Thank you!