What is the difference between isend and issend?

Question

Need clarification to my understanding of isend and issend as given in Send Types

My understanding is that isend will return once the send buffer is free, i.e. when

Answer 1

Correct. Obviously both of those will only be true when the request that you get back from the call to MPI_ISEND or MPI_ISSEND is completed via a MPI_WAIT* or MPI_TEST* function.