If the switch is set to Primary, the option card in the Option 2 header will be used for outputs 1-24.
If the switch is set to Secondary, the option card in the Option 2 header will be used for Outputs 25-48
The switch only affects the numbering for the Option 2 header. The Option 1 header will always be used for Outputs 1-24. In this way you could set the switch to Primary and have 2 option cards for circuits 1-24, or set the switch to Secondary and have one option card for 1-24 and one for 25-48.
If you have any questions or concerns about this information, please contact ETC Technical Services.