The areas associated with a stage in the global stage lifecycle only contain active item revisions. An item revision at a stage is only active when there are no newer revisions of the same item at that stage, or any later stage.
If the item revision becomes the active revision at the new stage, a copy of the item revision is automatically placed in the area(s) mapped to the new stage.
If the item revision was the active revision at the old stage, the item revision is deleted from the area(s) mapped to the old stage.
For example, if you deploy foo.c from UNIT TEST to SYSTEM TEST:
A copy of foo.c is placed in the areas associated with the SYSTEM TEST stage.
foo.c is deleted from the areas associated with the UNIT TEST stage.