Transformer Taps

this is from ACME electric, a transformer manufacturer.

What are taps and when are they used?

Taps are provided on some transformers on the high voltage winding to correct for high or low voltage conditions, and still deliver full rated output voltages at the secondary terminals.
Standard tap arrangements are at 2.5%and 5% of the rated primary voltage for both high and low voltage conditions.
For example, if the transformer has a 480 volt primary and the available line voltage is running at 504 volts, the primary should be connected to the 5% tap above normal in order that the secondary voltage be maintained at the proper rating.
The standard ASA and NEMA designation for taps are “ANFC” (above normal full capacity) and “BNFC” (below normal full capacity).