The Subject Wizard - Select System form lets you define which System your Subject belongs to.  You can not reassign an existing Subject to a different system.

Click on the Combo box button to see a list of available Systems, and then click on the name of the appropriate System to select it.

Once you have selected the System, click on the 'Next >' button to move on to the Subject Wizard - Select Smart Data Object form.

Click on the '< Back' button to return to the Subject Wizard - Name and Description form.

Click on the 'Cancel' button at any time to leave the Subject Wizard and return to DataPA SetUp without saving any changes.