Design calculation for the diameter-changing section of a container?

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I am designing a fixed-tube-sheet heat exchanger, in which one end of the tube box does not have a head and is instead designed as a tapered section (one end connected to the tube sheet and the other end to the column). However, when using SW6 to perform calculations for this fixed-tube-sheet heat exchanger, I used the calculation parameters for conical heads with flanges at both ends, as available in SW6’s component library. The calculation results showed that the stress levels were within acceptable limits, but it was indicated that the length of the reinforced sections of the cylinder and the cone shell needed to be more than 40 mm. What does this mean by the length of the reinforced sections of the cylinder and cone shell? Does it refer to the required length of straight sections in these components? But in the drawings made by the seniors, the straight sections of the diameter-changing parts are both 25 and 40. How should this be understood? Could someone please advise if there are any other calculation methods for the diameter-changing section?