Pretty sure it means that over the actual water opening cap, there is a second cap protecting the inner cap and the opening itself. This means that the outer ca

Pretty sure it means that over the actual water opening cap, there is a second cap protecting the inner cap and the opening itself. This means that the outer cap might get dirty on the way between home and water, but the opening itself doesn't, significantly reducing the risk of any dirt being washed in when filling in, or taking out, the water. Without it assorted dust and grime falling on the water opening cap in transit might over time work itself into the winding and much more easily get inside the container.