The HD Flex™ Equipment Rail Kit contains 6 rails and 12 labels to allow 6-port cassettes to be used in HD Flex Enclosures that are pre-configured for 4-port or 12-port cassettes. The HD Flex™ Equipment Rail Kit contains 6 rails and 12 labels to allow 6-port cassettes to be used in HD Flex Enclosures that are pre-configured for 4-port or 12-port cassettes.