Im stretching the meaning a bit but w/e
Dia is a for of light/bright/less of something/less substantial it is equal to its counterpart luna but its substance is different.
Dia is ment for light blows, speed and misdirection. Its something thats full but with a light substace, like light or air.
Luna is the counter part of Dia, its subtance is more tangible relating to more of a frontal approach.
Luna/Deep/Black its a form of direct assualt, Luna is ment for pure heavy force, strength, crippling blows, no deception involved its ment to hurt the enemy.
Zwei is just 2, or second form/version of something already there if its attached to a specific action or object. otherwise its just simple "2"
Luna and Dia are unique because one cannot trump the other, but someone who masters both can trump anyone else.