Exciters (aural/harmonic)
Exciters or aural harmonic exciters are in my opinion advanced forms of EQ but without having the ability to effect the low range of sound. Particular care needs to be taken when using exciters because they can easily thin out the entire sound of a composition as soon as it is turned on. This is why it is not really recommended for use in the mastering process.
For the sake of do it yourselfers not making mistakes by choosing to use this type of sound processing, it's is important to let it be known that using a exciter is not a wise decision. It is usually not found in the mastering chain because the EQ bands are sufficient enough. Exciters just add more brightness to the overall sound, but also has some effects on the harmonics, as well as the dynamics, but you want to stay away from thinning out the sound. Subtle distortion can be added with exciters like on a electric guitar track.
On that note, it is best to save the exciters for use on a single track in the mix like on the vocals, or background singers. They are also perfect for stand alone sounds that are dull in the mix and need special enhancing.
Use of exciters are fine as long as they are used sparingly, but are not necessary for the mastering process.