MPI procedures sometimes assign a special meaning to a special value of a basic type argument; e.g., tag is an integer-valued argument of point-to-point communication operations, with a special wild-card value, MPI_ANY_TAG. Such arguments will have a range of regular values, which is a proper subrange of the range of values of the corresponding basic type; special values (such as MPI_ANY_TAG) will be outside the regular range. The range of regular values, such as tag, can be queried using environmental inquiry functions (Chapter 7 of the MPI-1 document). The range of other values, such as source, depends on values given by other MPI routines (in the case of source it is the communicator size).
MPI also provides predefined named constant handles, such as MPI_COMM_WORLD.
All named constants, with the exceptions noted below for Fortran, can be used in initialization expressions or assignments. These constants do not change values during execution. Opaque objects accessed by constant handles are defined and do not change value between MPI initialization ( MPI_INIT) and MPI completion ( MPI_FINALIZE).
 
The constants that cannot be used in initialization expressions or  
assignments in Fortran are:  
 
MPI_BOTTOM MPI_STATUS_IGNORE MPI_STATUSES_IGNORE MPI_ERRCODES_IGNORE MPI_IN_PLACE MPI_ARGV_NULL MPI_ARGVS_NULL
 
In Fortran the implementation of these special constants may require the  
use of language constructs that are outside the Fortran  
standard. Using special values for the constants (e.g., by defining  
them through  parameter statements) is not possible because an  
implementation cannot distinguish these values from legal  
data. Typically, these constants are implemented as predefined  
static variables (e.g., a variable in an  MPI-declared  COMMON  
block), relying on the fact that the target compiler passes data by  
address.  Inside the subroutine, this address can be extracted by some  
mechanism outside the Fortran standard (e.g., by Fortran extensions or  
by implementing the function in C).  
 ( End of advice to implementors.)