A low-grade worker; a peon. A dumb person doing a dumb job, often in retail, food service, or warehousing.
"I'll get a couple of carbon units to come round tonight and take that junk away."
"One of the carbon units in Goods Inwards tripped over a cable and smashed his arm. "
The human operator of a machine, a computer, or any device in general that is having "problems" or "errors" or "is malfunctioning" in the eyes of that operator. In this case, the problems, errors, and malfunction is actually a problem with the operator and not the machine, computer, or device.
Customer: What was the problem with my computer?
Tech: There is a problem with the carbon unit. It will eventually resolve itself after extended hours of use. At least that is the hope.