The MRU (motion reference unit) records the heave, pitch, and roll of the vessel. Some models also record the heading of the vessel. It is sometimes called an IMU (inertial measurement unit) as well.
The MRU component does not have any attributes of its own in the Attributes window. However, it does contain the Heave, Pitch, and Roll components, and possibly the Heading component too, which have their own attributes in the Attributes window.