Final answer:
In a fixed enclosure, the PLC is known as a fixed or compact PLC, which is different from a modular PLC where modules can be added or removed.
Step-by-step explanation:
If all the units are in one fixed enclosure, the PLC is called a fixed or compact PLC. This type of PLC has all its components housed in a single unit or case, with no possibility of adding or removing modules. This is different from a modular PLC, where you can add or remove modules such as I/O, power supply, and others as needed for the application.