Flanged or other connected equipment with a restricted-area sealing means, at or near the face of its lower flange. NOTE Cross-over spools are also provided with suitable means to suspend and seal around an inner string of casing or tubing. A cross-over spool has a top connection with a pressure rating above that of the lower connection.
Source: API SPEC 6A, Specification for Wellhead and Christmas Tree Equipment, Twentieth Edition, October 2010 (Addendum November 2012). Global Standards