Center the actor inside the allocation
Snap to right or bottom side, leaving space to the left or top. For horizontal layouts, in right-to-left locales this should be reversed.
Stretch to cover the whole allocated space
Snap to left or top side, leaving space to the right or bottom. For horizontal layouts, in right-to-left locales this should be reversed.
Controls how a #ClutterActor should align itself inside the extra space assigned to it during the allocation.
Alignment only matters if the allocated space given to an actor is bigger than its natural size; for example, when the #ClutterActor:x-expand or the #ClutterActor:y-expand properties of #ClutterActor are set to %TRUE.