The new signals are usually calculated at specific musical intervals to simulate vocal harmonies. Professional versions allow the user to 'capture' the unwanted noise and use it as a template for the reduction process. Parametric or 'sweep' equalization enables the user to adjust the gain of a specific frequency. High-end versions have multiband parametrics for low, mid and high band frequencies. Advanced plugins have the ability to retune an signal without altering the original duration.
Professional versions give the user facilities to control the ambient nature of the effect.
Whilst this list is not exhaustive , the majority of alternative plugins available are variations of theses basic types.