In a D-Wave QPU, the set of qubits and couplers that are available for computation is known as the working graph.
The yield of a working graph is typically less than the total number of qubits and couplers (devices) that are fabricated and physically present in the QPU. Manufacturing variations and the need to prepare the QPU to operate at cryogenic temperatures in a low–magnetic field environment limits the yield. These variations are minimized through an extensive calibration process that attempts to bring all of these analog devices into a consistent parametric regime.
See also: What are the Properties of this QPU?