Three other variable prefixes are occasionally used in Csound instruments. The S- prefix denotes a text string variable. The w- and f- prefixes are used with spectral data types. Spectral processing is beyond the scope of Csound Power! An introduction can be found in The Canonical Csound Reference Manual, on the “Tools for Real-Time Spectral Processing” and “Non-Standard Spectral Processing” pages.
Both S- and f- variables have global versions (gS- and gf-, respectively).