Rms traces the rms of a signal. Gain modifies a signal so that it has a specified rms power. Balance is a combination of the two which modifies a signal so that it has rms power equal to that of a compare signal.
Balance is useful for restoring power to a signal after it has passed through a filter.
Balance and gain return a modified signal, rms returns the rms of the signal. Extreme cases will cause spikes in the audio - if this occurs try using a gain multiplier to approximate the target rms before applying balance or gain to it. It is a good idea to use a multiplier in any case because balance and gain work better the less they have to gain.