tag, upper bound
Tag values range from 0 to the value returned for MPI_TAG_UB,
MPI_TAG_UB
inclusive.
These values are guaranteed to be unchanging during the execution of an MPI
program.
In addition, the tag upper bound value must be at least 32767.
An MPI implementation is free to make the value of MPI_TAG_UB larger
than this;
for example, the value 
is also a legal value for
MPI_TAG_UB (on a system where this value is a legal int or
INTEGER value).
The attribute MPI_TAG_UB has the same value on all MPI_TAG_UB processes in the group of MPI_COMM_WORLD.