B cars were used in the middle of trains. These cars did not have automatic controls, and relied on lead and trailing units for their operation in revenue service. They ran under manual control as single units in the yard.
For very technical details on the original A and B cars, see the page Rohr: Background on the BART Car.
All surviving B cars were later rebuilt into B2 cars. See the page B2 cars for their lives after 2002, and the rebuilding page for information on the rebuilding processs.
In later years, certain A cars were converted into B cars (800 series). See the page A to B conversion page on the conversion story.