"X40" membrane was developed first, prior to X50.  The "X50" membrane was then developed to allow a much higher amount of gas to pass through; this is why it is best used for CO2 reduction.  X40 can reduce CO2; it is just not as efficient (but may be evaluated on a case-by-case basis). X50 is optimized to remove a larger amount of CO2;  this membrane has a higher porosity and the wall is slightly thinner. This allows higher transport of soluble gas (in X50) through and out of the hollow fiber. In X50 the Henry’s Law ‘gradient’ is reestablished incrementally faster than in X40. The X50 was developed to deal with high levels of dissolved gases such as CO2, which is usually much higher than typical dissolved oxygen levels. SP (superphobic) membranes are non-porous but gas-permeable, and are used for degassing and debubbling low surface tension fluids, like inks.