This is the name of the colony address space that the NFS client is started in. The default name is MVSNFSC. The name of the NFS Client address space must match the name of the cataloged procedure in SYS1.PROCLIB that specifies the first program to run in the address space. It must also match the RACF user ID for the cataloged procedure. And you need a RACF STARTED Class profile for the cataloged procedure.
A sample cataloged procedure for the address space is shipped with NFS; it is documented in NFS Customization and Operation.