For physics measurements with polarized colliding beams, beam polarizations and relative luminosities must both be determined. Predictions for spin observables of many interesting physics processes at RHIC are quite small in magnitude. This requires high statistics measurements of relative luminosities and careful control of systematic errors. Discussions about the polarized beams at RHIC often presume that the polarization and intensity of each bunch within a will be known quite well from measurements by the RHIC polarimeters. The purpose of this note is to give a description of the knowledge that can actually be obtained from these polarimeters.