The capacity test discharges the bank at constant current from full charge down to the final discharge voltage, timing how long it takes. Current times time gives the real amp-hours, compared against the nameplate value corrected for temperature.
It is the only test that truly answers the question that matters in a backup system: how long the bank will hold when the supply fails. Float voltage, internal impedance and conductance are useful, quick indicators, but none replaces a real discharge.
IEEE 450 for vented batteries and IEEE 1188 for valve-regulated ones set the frequency and the criterion: end of life is taken as capacity falling below 80 % of rating, because beyond that point the decline accelerates. The usual discharge current corresponds to the C/8 or C/10 rate.