An MCU is a combination of two or more lines (often of different functional types), including hydraulic lines, electrical cables, fibre optic cables and sometimes small-bore service lines (e.g. chemical injection lines). An MCU is typically armoured with steel wire, but is still sufficiently flexible to be deployed from a reel or a carousel on an installation vessel. Depending on manufacturing and/or transport constraints, an MCU may have dry splices in it at various points along its length, which are typically made prior to loadout of the umbilical onto the installation vessel.
Source: API RP 17A, Design and Operation of Subsea Production Systems—General Requirements and Recommendations, Fourth Edition, Reaffirmed 2011. Global Standards