A spool used to connect drill-through equipment with different end connections, nominal size designation and/or pressure ratings to each other.
Source: API STD 53, Blowout Prevention Equipment Systems for Drilling Wells, Upstream Segment, Fourth Edition, November 2012. Global Standards
Connects blowout preventers of different sizes or pressure ratings to the casing head.
Source: IADC UBO / MPD Glossary, December 2011. Global Standards