title: When to use tags when sending and receiving messages in MPI?
body: <p>I'm not sure when I have to use different numbers for the <strong>tag</strong> field in MPI send, receive calls. I've read <a href="http://mpitutorial.com/tutorials/mpi-send-and-receive/" rel="noreferrer">this</a>, but I can't understand it.</p>
  
  <blockquote>
    <p>Sometimes there are cases when A might have to send many different
    types of messages to B. Instead of B having to go through extra
    measures to differentiate all these messages, MPI allows senders and
    receivers to also specify message IDs with the message (known as
    tags). When process B only requests a message with a certain tag
    number, messages with different tags will be buffered by the network
    until B is ready for them.</p>
  </blockquote>
  
  <p>Do I have to use tags, for example, when I have multiple calls "isend" (with different tags) from process A and only 1 call to "ireceive" in process B?</p>