What is the Significance of US Folder?

It is nothing but language specification by default it is in american language.

We can have multiple languages folders  based on installed languages.

from backend we can get it from FND_LANGUAGES Table and column INSTALLED_FLAG I,B,D

              I--INSTALLED,
              B--BASE,
              D--DISABLE

select language_code,nls_language from fnd_languages where installed_flag like 'B'