In addition to data, messages carry information that is used to distinguish and selectively receive them. This information consists of a fixed number of fields, which we collectively call the message envelope. These fields are message envelope source, destination, tag, and communicator.
sourcedestinationtagcommunicator The message source is implicitly determined by the identity of the message source message sender. The other fields are specified by arguments in the send operation.
The comm argument specifies the communicator used for
communicator
the send operation. The communicator is a local object that
represents a communication domain. A communication domain is a
communication domain
global, distributed structure that allows processes in a group
groupprocess group
to communicate with each other, or to communicate with processes in another
group. A communication domain of the first type (communication within a
group) is represented by
an intracommunicator, whereas a communication domain of the second type
intracommunicator
(communication between groups) is represented by an intercommunicator.
intercommunicator
Processes
in a group are ordered, and are identified by their integer rank.
rankprocess rank
Processes may participate in several communication domains; distinct
communication
domains may have partially or even completely overlapping groups of processes.
Each communication domain supports a disjoint stream of communications.
Thus, a process may be able to communicate with another process via two distinct
communication domains, using two distinct communicators. The same
process may be identified by a different rank in the two domains;
and communications in the two domains do not interfere.
MPI applications begin with a default communication domain that includes
all processes (of this parallel job); the default communicator
MPI_COMM_WORLD represents this communication domain.
MPI_COMM_WORLD
Communicators are explained further in Chapter 
.
The message destination is specified by the dest destinationmessage destination argument. The range of valid values for dest is 0,...,n-1, where n is the number of processes in the group. This range includes the rank of the sender: if comm is an intracommunicator, then a process may send a message to itself. If the communicator is an intercommunicator, then destinations are identified by their rank in the remote group.
The integer-valued message tag is specified by the tag argument.
tagmessage tag
This integer can be used by the application to distinguish messages.
The range of valid tag values is 0,...,UB, where the value of UB is
implementation dependent. It is found by querying the
value of the attribute MPI_TAG_UB, as
MPI_TAG_UB
described in Chapter .
MPI requires that UB be no less than 32767.