The MU script is automatically attached to all objects created by a Source. As a user, you don’t need to interact with the MU script at all, except when the script itself publishes information or requests some input.
This is the Inspector Window for an MU script:
Each MU automatically receives a global ID (this is globally unique) and a source ID (this is only unique to the Source).
You can get information about the joint (Fixed To Joint) that the MU is currently attached to (when it is gripped through the physics function) or the component it has been attached to as a sub-component by using (Gripped By). See also the description of Grip how MUs can be gripped.
With Loaded On you detect, if the MU is currently loaded on another MU after adding it as a sub-component. The Parent Before Grip defines the parent of the MU prior to the release into the sub-group of the next gripping object. MUs that are already loaded can be checked from Loaded Mus.