The codes refer to the diseases that the particular tomato plant has resistance to.
V- verticillum wilt
F- fusariuim milt (ff- races 1 and 2 fff-races 1,2,3)
N-nematodes
T-tobacco mosaic virus
A-alternaria stem canker
ST- stemphylium grey leaf spot
TSWV- tomato spotted wilt virus
The Code...