Problem:

A new run-time tunable, use_mfextmap_migration_tool, has been introduced to control whether the MFEXTMAP Migration Tool is invoked when ddnames are resolved via the mfextmap.dat file. Set the tunable to a value of "false" to prevent the MFEXTMAP Migration Tool from being invoked. The default is "true".

You need at least Mainframe Express v3.1 Wrapack3 to enable this feature

Resolution: