Understanding 3ph Transformer Specs
FCAN and FCBN designate voltage adjustment taps at Full Capacity Above Normal voltage and Full Capacity Below Normal voltage. That indicates that the voltage can be adjusted by a designated percentage above and below the normal primary or secondary voltage. The transformer can be operated at its full capacity with any tap setting.

For the example transformer, the taps would be connected to terminals 3 if the primary voltage is the normal 480 volts. If the voltage is 2.5% or 5% above 480 volts, the taps would be connected to terminals 2 or 1 respectively. If the voltage is below 480 volts, the taps would be connected to terminals 3, 4, 6, or 7 for 2.5%, %, 7.5% and 10% below 480 volts.

For all primary voltages corresponding to the tap positions, the secondary voltage will be 208/120 volts and the capacity will be 30 kVA.

The transformer can supply a combination of linear and non-liner loads. The taps have no bearing on that. Theoretically, the transformer should be able to supply more kVA for linear loads than for non-linear. However "non-linear" implies an unspecified degree of harmonic content in the load current. The transformer may have a specification that quantifies harmonic current distortion vs. kVA rating.