This setting governs whether the Engine allows a new case to be created without an expected or actual end date.
If you change this setting for your product, then it will only affect the processing of new cases created in the future; all existing cases which have already been created and stored are entirely unaffected. Thus it is possible for a product not to allow the creation of open-ended cases, yet have cases registered against which are open-ended (because those cases were registered at a time in the past when the product did support open-ended cases).