What is the difference between Water Holding Capacity, Field Capacity and TAWC?
Okay Water Holding Capacity(WHC) is how much water the soil has in it at total saturation, if you filled every soil pore space completely with water.

Field Capacity(FC), if I remember correctly from first year, is shorthand for the water that stays in the soil after surface runoff and groundflow has ceased; the sub-saturation "ground state" of the moist soil if you will.

Soil Water(SW) would appear based on the excerpt provided to be referring to the percentage of WHC present at time of measurement.

You've also asked about Total Available Water Capacity(TAWC) which is the water that can be accessed by plants, this is the water stored in macro, as opposed to micro, pores in the soil (it's more complex than that but we always worked it as a two grade problem; pores big enough for plant roots and pores too small for them). Usually to quantify this figure WHC, or FC depending on what's of interest, is multipled by the percentage of plant accessible pore spaces in the soil, this gives the volume of water plants can access before they start to wilt.

Please note I'm mostly working from my memory of an entry level class I took nearly a decade ago so this may not be perfect, but it it does check against the research material I quickly skimmed while writing.