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The aggregate spin number is used in the SPFIT and SPCAT programs whenver more than six quantum numbers are needed to descibe a state – even if some of the quantum numbers are redundant. It can be decoded using information provided in the .fit file or the .out file after the list of parameters in both cases. The NH₂ example is used, in which S = 1/2, I_N = 1, and I_H = 1/2 for both H nuclei and N + S = J; J + I_N = F₁; I_H1 + I_H2 = I_tot; and F₁ + I_tot = F.

Please note: The coding will be the same for equivalent spin combinations, meaning one spin of 1/2, one spin of 1, and two equivalent spins of 1/2. The coding will be different for other spin combinations, e. g. two spins of 1/2 and two equivalent spins of 1/2 with PH₂ as an example or one spin of 1, one spin of 1/2 and two equivalent spins of 1/2 with ¹³CH₂ as an example.

BLOCK - WT - SYM - V - TSP - N  -  other quanta  (rel. to F=0 )
    1    1    0    0    0  -2.5  -2.0  -1.0   1.0

TSP is the aggregate spin number, here 0. Subsequently, N and the other quanta (J, F₁, and I_tot) are given relative to F = 0.
If we take, for example, line number 12:

  3  3  1  0  0  6  4  1  3  0  0  7   11686.80

we see that for the upper state we have N = 3, hence F = 3 + 2.5 = 6 – 0.5 = 5.5, J = 3.5, F₁ = 4.5, and finally I_tot = 1.0 is redundant.
Similarly for the upper state: N = 4, hence F = 6.5, J = 4.5, F₁ = 5.5, and finally I_tot = 1.0 is again redundant.
For TSP = 1 we find:

    2    1    1    0    1  -1.5  -1.0   0.0   0.0

For example in line 21 there is TSP = 1 twice:

  5  2  3  0  1  7  6  1  6  0  1  8    8749.68

For the upper and lower state, respectively:
N = 5, N = 5.5, F₁ = 6.5, I_tot = 0.0, and F = 6.5.
N = 6, N = 6.5, F₁ = 7.5, I_tot = 0.0, and F = 7.5.

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