When a configuration interaction calculation is done, all spin states are calculated simultaneously, either for component of spin = 0 (for even electron systems) or 1/2 (for odd electron systems). When only singlet states are of interest, then SINGLET can be specified, and all other spin states, while calculated, are ignored in the choice of root to be used.
Note that while almost every even-electron system will have a singlet ground state, SINGLET should still be specified if the desired state must be a singlet.
See also MS=n, DOUBLET, TRIPLET, QUARTET, QUINTET, SEXTET, SEPTET, OCTET, and NONET.
If the UHF method is used, then if SINGLET is present, the system will be checked to verify that the number of electrons is even. If it is, then SINGLET will be ignored, otherwise the calculation will be stopped, and an error message printed.