What is the difference of Additonal Include Directories and Additional #using Directories?

In Visual Studio (Visual C++)Project setting we able see these settings:

• Additional Include Directories (/I[path] compiler switch)
• Additional #using Directories (/AI[path] compiler switch)

I want to know when to use these settings appropriately and what the main differences between these settings and how they affect on visual c++ linker and build time?

Please look at here -Visual Studio Property page

#using is for C++/CLI, not for standard C++.

A directory to search to resolve file references passed to the #using Directive directive.

assembly_A.cpp

// compile with: /clr /LD  
public ref class A {};  

assembly_B.cpp

// compile with: /clr /LD  
#using "assembly_A.dll"  
public ref class B {  
public:  
   void Test(A a) {}  
   void Test() {}  
};  

The option /AI[path] would set a search path where assembly_A.dll is placed.

More official info: VCCLCompilerTool.AdditionalUsingDirectories Property