A battery does not deliver the same amp-hours at every current: the faster it is discharged, the less capacity it gives before reaching the final voltage. The German engineer Wilhelm Peukert described that loss in 1897 with a simple law, In × t = constant, whose exponent n bears his name. With the rated capacity C given at an H-hour rate, the discharge time at a current I is t = H × (C / (H × I))n.
With n = 1 the battery would be ideal and its capacity would not depend on current. In vented lead-acid the exponent is usually between 1.2 and 1.3, in valve-regulated batteries (AGM and gel) between 1.05 and 1.15, and in lithium-ion very close to 1. It is worked out from two points of the manufacturer's table, two currents I1 and I2 with their times t1 and t2: n = log(t2/t1) / log(I1/I2). The exponent rises with ageing and changes with temperature.
It is a useful approximation for estimating autonomy, but it departs from reality at very fast or very slow discharges and ignores the chosen final voltage. That is why IEEE 485 sizes with the manufacturer's curves rather than with Peukert. The battery calculator uses it for autonomy when those curves are not available.