When ZAFT builds three new mobile suits based on the stolen MS units GAT-X102 Duel, GAT-X103 Buster], GAT-X207 Blitz and GAT-X303 Aegis, they develop the ZGMF-X_A series and re-designate the operating system for those suits as Generation Unsubdued Nuclear Drive Assault Module Complex (or GUNDAM Complex). After the First Bloody Valentine War, the OS is updated to support the Deuterion Beam Energy Supply System that the newer ZAFT suits use, and the system the ZGMF-X_S series uses is re-designated as Generation Unrestricted Network Drive Assault Module . Later in the war a third version of the OS, Gunnery United Nuclear-Deutrion Advanced Maneuver System, is used in the new ZGMF-X42S Destiny and ZGMF-X666S Legend. ZAFT experimental units generally have their numerical designation prefaced with X followed by a two or three digit type code and a letter designating their stage of development: A for First Stage, S for Second Stage and T for Third stage suits. For Second Stage units, the two-digit series designators include a type number (first digit) and a series number (second digit), so the ZGMF-X56S Impulse is Type 5, Series 6. The only exceptions are the ZGMF-X19A Infinite Justice and ZGMF-X20A Strike Freedom due to the fact that they aren t constructed by ZAFT and are resurrections of the earlier prototypes. The Type numbers significance for X_S-series Second Stage units is as follows: 1: None; only first generations were given this number, but their model numbers are in sequence 2: Transforming Air space-use units 3: Underwater-use units 4: General all-purpose-use units 5: Silhouette system units 6: DRAGOON system units 7: Unknown 8: Ground-use units