"Meaning of Designation FFG(X)
In the program designation FFG(X), FF means frigate,11 G means guided-missile ship (indicating a ship equipped with an area-defense AAW system),12 and (X) indicates that the design of the ship has not yet been determined. FFG(X) thus means a guided-missile frigate whose design has not yet been determined.13"
https://www.everycrsreport.com/files/20171208_R44972_938afb756a3856cb6bbe687367fd51eaf98beb14.html#_Toc500509536