If you are looking at a Visu diagram for a Laguna II or Megane II, you will see a giant rectangle labeled (or BSI later on). This is the "intelligent" fusebox. In Visu, the UCH is treated like a mini-computer.
The most daunting aspect for beginners is the alphanumeric soup. You will see codes like B , C , M , and numbers like 120 or 225 . renault visu wiring diagrams