It is simply recording samples in another language as to allow users to use those samples to quickly make the UTAU sing in that language, as opposed to slaving over a "to" and an "u" to make "two" because the only other option is to use "tsu." Or, with Japanese UTAUloids, use 'tu' as a substitute for 'two'.

The various methods for English banks are detailed in An English Bank Creating An English Bank