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This option is introduced to help users to pick a valid calculation method. Because for some parameters certain calculation methods do not make sense.
For instance for a parameter "Precipitation Intensity" with type "mean" and unit "mm/h" it does not make sense to chose calculation method sum because intensity is not meant to be summed. If in 1 hour every 15 minutes the intensity was measured at 5, 10, 0 and 5 and you calculate the sum over that hour you get an intensity of 20 mm/h, which is wrong. But for a parameter "Precipitation" with type "accumulative" and unit "mm" this does make sense. If in 1 hour every 15 minutes the precipitation was measured at 5, 10, 0 and 5 and you calculate the sum over that hour you get a total precipitation of 20 mm, which is perfectly fine.
For a parameter "Gate position" with type "instantanious" it could be that the only valid values are 0 or 1, meaning closed and open. Calculating a sum or a mean will not make sense because it can result in invalid data above 1 or between 0 and 1.