I have some questions.
What does the DT stand for on the neerdom key?
What does the U stand for in the 1u, 1.25 u, 1,75u and so on
What is the Mono Legend for?
What is the difference between standard and pro modifiers?
What is Extra homing for?
Is the Icon modifiers your design? is there any logic? they look great! 
Everything looks awesome! just new on the keycaps world, Sorry for extremely stupid questions 
The u is for units. It indicates the size of the key cap.
The typical key cap is 1u. These are your alpha keys. Other sizes are usually a factor of 1u. So a Win/OS key in a Tenkeyless (TKL) layout is said to be 1.5u, or 1.5x the horizontal length of an alpha key. The spacebar is - I believe - 6.25u. This is a rough standard used in the keyboard enthusiast community. You'll notice some keyboards (especially something like the MS Natural Keyboard line) don't adhere to any particular standard.
Edit: For your other questions...
DT is Deskthority, a web site similar to geekhack (/ducks).
http://deskthority.net/Monolegend is just for people who don't want top & bottom legends printed on their keys. They know that the $ sign is on the #4 key, so they don't need to see a "4" and a "$" printed on the same key. It's a personal preference.
Pro modifiers are for people with special boards like HHKB and Pure -- I don't have sufficient knowledge of these to give you an accurate list.
Extra home row is for people who use a non-QWERTY alpha key layout like Dvorak. The F and J keys are typically made with a homing "bump" or a deep dish -- the key cap is different so your fingers can find the home row easier. So guys with a Dvorak layout need different homing keys because F and J are not on their home row!
I can't really answer your last question.
