You can toggle between the two output specifications by typing an 'O' on the command line of the RC/M Migration Analysis panel.
COMMAND ===> o
'o' is an abbreviation of the OUTPUT line command which can also be used.
Alternatively you can set the default behavior to control the specification of the Analysis output, this is done via the RC/Migrator Profile parameters
On the RC/Migrator Main Menu
0 - Profile
2 - RC/Migrator Profile Variables
On the RC/M Global Operational Parameters panel
UPDATE OUTPUT SPECIFICATIONS ===> y
ANALYSIS OUTPUT TO PDS ===> Y
This sets the default to write the strategy analysis output to a PDS.