Think of the soil moisture characteristic curve, which plots how tightly a soil holds water at different moisture contents. Certain points on that curve get special names because they mark real changes in soil behaviour.
Saturation capacity marks the top of the curve, where every pore is full. Field capacity marks the point where free drainage has essentially stopped. Permanent wilting percentage marks the point where the remaining water is held so tightly that roots cannot pull it out. These three are properties fixed by the soil's own pore structure and texture, so they are rightly called soil moisture constants.
Evapo-transpiration does not belong on this list. It is the combined loss of water through evaporation from the soil surface and transpiration through plants, and its value keeps changing with sunshine, wind, temperature and the crop growing on the field. A quantity that shifts with the weather cannot be a fixed constant of the soil.
\[\boxed{\text{Evapo-transpiration}}\]