The cement density is used in the calculation of the total aggregate content and of the concrete density.
If you have selected in Settings to calculate the concrete density by using the DOE Method approximation, Firstmix assumes a cement relative density of approximately 3.12.
Otherwise Firstmix uses the cement density specified for the selected cement. The default cement files specify a density of 3120 kg/m3 (5258.9lb/yd3), but the user can change this with the Cement Properties Dialog.
Strictly the DOE Method requires that the cement density specified should be the absolute density. However the bulk and absolute densities of cement do not differ greatly in practice, since cement is usually well-graded and must be kept dry, and in any case the precise cement density is not critical in mix design. It is therefore reasonable to use the bulk density.