The rank of the talent dictates the actual strength of the talent while the tier dictates the usability. For instance an S rank, grey tier talent for attack may be the ability to double the damage you would deal when you use all your strength in a single attack, whereas an F rank, rainbow tier talent for attack my be that your damage is increased by 5% on all hits no matter what.
Rank
F = The weakest talents or ones with massive drawbacks, about 11% of people in the galaxy
E = Talents that are barely worth having, about 22% of people in the galaxy
D = Talents that place you solidly in the middle tier of humanity, about 35% of people in the galaxy
C = Talents that may add bonuses other than purely what it says, about, about 18% of people in the galaxy
B = Talents that add bonuses other than what it purely says, about 8% of people in the galaxy
A = Talents that are nearly unstoppable, about 6% of people in the galaxy
S = Talents viewed as the peak of what a talent can do, under 0.000005% of people in the galaxy
X = Reality breaking talents, less than 100 people in the galaxy have one
Tier
Grey = The talent can only be used in very precise circumstances
Yellow = The talent has very severe restrictions
Green = The talent has severe restrictions
Blue = The talent has moderate restrictions
Pink = The talent has very few restrictions
Purple = The talent has almost no restrictions
Gold = The talent has no restrictions
Silver = The talent can do slightly more than what it says
Orange = The talent can do more than what it says
Rainbow = The talent can do much more than what it says
Type
Offensive: Offensive talents are the most diverse, and able to be almost anything
Defensive: Defensive talents are the second most diverse, they often appear to break the laws of physics
Supportive: Supportive talents are often healing based, however can be to give buffs and debuffs. They are some of the most valuable people in the war
Utility: Utility talents can be anything from boosting crop quality and quantity, to manipulating the earth. They are the main reason humanity can devote so many people to the war effort
Branch: Branch talents are incredibly rare, however lower tier branch talents are deemed as useless due to the lack of an affinity bonus. Branch talents increase someones knowledge, experience and intuition towards specific things such as swords, fire magic, etc.