Why & qualifier is used with assignment operator?

My question is in reference to the following links:

Why declare a deleted assignment operators with the ref-qualifier &

Should I use lvalue reference qualifiers for assignment operators?

I could not follow the posts. Could somebody explain:

1. What does the & at the end in the given code snippet actually do? auto operator=(const string& rhs) &;.
2. Why does the & at the end of this given code snippet is redundant? (Please also confirm if its just redundant or it is problematic) auto operator=(const string& rhs) & = delete;
3. Is this statements is valid, auto operator=(const string&& rhs) &&;?

If qualifier & prevents copy assignment of rvalue, Is it not already taken care by the type of input parameter specified? (const string& rhs).

In the following example, with or without qualifier, I get same result for rvalue assignment.

class copy_assignment{
   public: 
   copy_assignment& operator=(copy_assignment& a) & {cout << "copied"; return a;}; 
   copy_assignment* get_ca(){return new(copy_assignment);}
};

int main()
{
    copy_assignment a, b;
    a = *b.get_ca();
}