Check the TAG instructions etc for more detail.

IMO,
water grains, <water> vapor pressure, etc. are all related measurements refering to the amount of water vapor in the air. And alcohol fuels are more sensitive to this than gas fuels. Relative humidity doesn't capture the entire picture, since it is a ratio of the maximum amount of water vapor possible at a given temperature to the actual amount of water vapor. So if the temperature drops and the total water gains or water vapor pressure stays the same, the relative humidity still goes up since colder air cannot hold as much water vapor.

I think your TAG has all the information needed (Temp, RH, Baro). With 3 of the 4, it can calculate the other and even density altitude; in this case it can calculate dew point, water grains, or vapor pressure. Again check the instructions, but I bet it just wants to know which prediction method to use based on the fuel. One is temperature/DA biased and the other is humidity biased.

When the barometric pressure is 29.92"Hg (for example), the amount of humidity (water vapor) in the air is part of that (partial pressure of water vapor). So it might be 29.21"Hg of air pressure and 0.71"Hg of water vapor pressure.