Industrial outdoor storage (IOS) is industrial-zoned land primarily used for the storage of trucks, trailers, vans, large equipment, containers and materials. The land can be improved by installing pavement, perimeter fencing and flood lighting. The defining factor of IOS, as compared with traditional enclosed industrial, is that the primary value is derived from the open, horizontal outdoor space, rather than a vertical building. This is reflected in the way IOS properties are rented on a per acre basis as opposed to per square foot. Any enclosed spaces on an IOS property serve ancillary purposes, such as truck maintenance and repair, loading and unloading, or back-office needs, and typically make up less than 20 percent of the property.