The Soundex(s) command implements the classical Soundex algorithm.

The Soundex algorithm is intended to hash words into a small space by using a model which approximates the sound of the word when spoken by an English speaker.  Each word is reduced to a four character string (a Soundex key), where the first character is an uppercase letter and the remaining three characters are digits. Soundex keys have the property that words, which are pronounced similarly, produce the same soundex key and can thus be used to simplify searches in databases where the pronunciation but not the spelling is known.

Knuth, Donald. The Art of Computer Programming, Volume 3: Sorting and Searching. Reading, Massachusetts: Addison-Wesley, 1973, pp. 391-392.