In this video top portion (Part9) is a cavity block. Bottom portion (Part8) is a core block. Part that is going to molded is named as Component.
Bottom portion (Part8) and molding part (Component) are used in the Design Component Box of the Property Manager to a create cavity in the Top portion (Part9). Even though cavity is going to be created in Part9 (top portion) that is not involved in cavity creating process. How is it possible?