Setting the effectivity date is done in a revisioning workspace that is used to revise/release the item: for example, a change order. If effectivity was enabled in the Lifecycle map for the target state of this item, you can choose an effective date for this item.
Set Effectivity
Even if effectivity is not turned on, an effectivity date is automatically selected for the item as the release date of the change order. This means that ALL items in the system have an effectivity date - either chosen or implied by the release date of the change order.
Note: Only a start effectivity date is selected. The end effectivity date is assumed to be the day before the next revision goes into effect. While this may be somewhat limiting in some cases, it ensures that there is continuity of effectivity as well as no overlapping effectivities of different revisions of the same item.