[VERY SIMPLE] Difference between writing Using X and Using X.Y

Hey! I have a simple question about libraries.

Could you please elaborate why if I want to use, say Unity UI, I have to add another line of library code at the top?

Why must I add “using UnityEngine.something (in our case “.UI”);”

I assume ALL libraries for UnityEngine have already been loaded by “using UnityEngine;” statement. The dot indicates the it should inherit properties from what stands before, or am I wrong?

Thank you in advance!

http://stackoverflow.com/questions/9023465/importing-nested-namespaces-automatically-in-c-sharp

Thank you very much!