What is a nested name specifier?

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I want to know what exactly is a nested name specifier? I looked up in the draft but I could understand the grammar as I haven’t taken any Compiler Design classes yet.

void S(){}

struct S{
   S(){cout << 1;}
   void f(){}
   static const int x = 0;
}; 

int main(){ 
   struct S *p = new struct ::S;  
   p->::S::f();

   S::x;  

   ::S(); // Is ::S a nested name specifier?
   delete p;
}