What is ref in Erlang?

Question

Going through this link

To identify a process,we would be using a Pid. When should a ref be used?

I see this often while message sending/receiving but unable

Answer 1

because ref is unique which can identify received message.

for example:

A,B two Actors, A want to known B's age

there are 2 solutions:

Method 1:  A send Msg({age}) to B,then B send result Msg({age,10})
but,before A receive this Msg,Actor C send Msg{age,20} to A,
then A got the wrong answer

Method 2: A send Msg({age,Ref}) to B,
no one knowns the Ref except A and B,
then A got the right answer from B.