Why: The RBD models allow calculation of system reliability based on knowing/assuming failure details of the components starting with the least component and growing the model to the greatest system to predict performance from the elements.
When: RBDs are used in upfront designs as a performance parameter and after the system is constructed to ferret out poor performing blocks that limit the system performance.
Where: Frequently used as a trade-off tool to search for the lowest long cost of ownership and to help sell alternative courses of action for moderating the effects of reliability issues or overcoming the poor performance by alternative designs where the results can be calculated before building the system as the results of the calculations provide knowledge about availability, maintenance interventions required for failures, and the number of spare parts required to sustain operations. For other definitions see MIL-HDBK-338, section 4 and 6.
These definitions are written by H. Paul Barringer and are also posted on his web site at www.barringer1.com