Standard full capacity taps are usually +(2) 2-1/2% abive normal and (4) 2-1/2% bellow normal. +5%, +2-1/2%, 0, -2-1/2%, -5%, -7-1/2%, -10%. Again, these are standard taps but manufacturers often offer a different combination of these taps.
When a transformer is installed the primary voltage available is often lower the the transformer pri. NP voltage. So "bellow" normal taps are provided to match the supply voltage so the the secondary voltage will be correct. If the transformer primary is 480v, a 2-1/2%FCBN tap will correct fort supply voltages 2-1/2% below 480v, etc.
It's not to often that a primary voltage is on the high side.