About the choice of the declations of `std::vector::push_back`

There two declarations for std::vector::push_back.I understand rvalue and lvalue to some degree. As far as i know, almost all types(T&、T&&、T) could be converted to const T&, so which one does the compiler choose when different types of object passed to std::vector::push?

I am a novice in C++.Though i thought over and over, i still could not get the idea.It would be better if you could give me some simple examples to make it clear.I would be greateful to have some help with this question.

As per the documatation(http://www.cplusplus.com/reference/vector/vector/push_back/), which says that:

void push_back (const value_type& val);

void push_back (value_type&& val);

Adds a new element at the end of the vector, after its current last element. The content of val is copied (or moved) to the new element.