The hamiltonian is constructed and diagonalized explicitly in the primary configuration space, which can be selected with the PSPACE card. The coefficients of the remaining configurations (Q-space) are optimized iteratively using the P-space wavefunction as zeroth order approximation.
If thresh is nonzero, it is a threshold for automatically selecting all configurations as P-space configurations which have energies less then , where emin is the lowest energy of all configurations. Further P-space configurations can be specified using CON cards, which must follow immediately after the PSPACE card. These are merged with the ones selected according to the threshold. Automatic selection can be avoided by specifying a very small threshold. There is a sensible default value for thresh (0.4), so you usually don't need a pspace card in your input. Furthermore, if the number of configurations in the MCSCF is less than 20, all configurations go into the P-space unless you give a PSPACE card in the input.
A P-space threshold defined on a PSPACE card before the first WF (or STATE, WEIGHT, SELECT, PUNCSF if WF is not given) card is global, i.e., valid for all state symmetries. State-specific thresholds can be defined by placing a PSPACE card after the corresponding WF card. In the latter case the PSPACE card can be followed by CON cards, which define state-specific P-space configurations.