Meaning of letters near Armor and Weapon in "Monster Hunter Freedom Unite"

I sometimes see letters in armor names and weapon names in Monster Hunter Freedom Unite:

• Buster Blade G
• Diablo D Armor
• Mafumofu S Armor
• Vespoid U Armor
• Blango X Armor
• White Fatalis Z Armor

I am guessing G is for "G-Rank", D is for "Dark", but I was wondering about the meaning of S, U, X and Z letters.

Thanks in advance.

G weapons are sets of weapons that similar to ones you get early in the game (e.g. the Buster Blade and Buster Blade G), but have G-ranked power/sharpness.

The letters S, U, X, and Z in armor names refer to them being High Rank (S/U) and G Rank (X/Z) variations of the Low Rank (regular/D) armors, usually having vastly different, higher base defense, and/or better/more skills and slots than their previous iterations.