There have been a lot of queries about this. These are settings in configure.user that allow shrinking memory on each processor to allow MM5 scaling in memory.

An explanation is online at:

http://box.mmm.ucar.edu/mm5/mpp/PROCMIN_NSEW.html

For additional information on the configure.user file see:

http://box.mmm.ucar.edu/mm5/mpp/configure.html