You can use ATG Campaign Optimizer to test content in both active slots (slots that can issue their own requests for content items) and passive slots (slots that do not issue requests for content items).

ATG Campaign Optimizer uses the FillSlot action to fill a slot with test content. If a slot is not registered as a Nucleus component and filled with the FillSlot action – for example, if the slot is created as a property of a JMS message – then the test content won’t be visible during the test. The slot continues to display its original content.

For more information about slots and targeters, see the ATG Page Developer's Guide. For details about the FillSlot action, see the ATG Personalization Programming Guide.