A multi-tiered, time-sequenced, step-by-step flow diagram of a system's functional flow is known as a functional flow block diagram (FFBD). In this sense, the term "functional" is distinct from its application in functional programming or mathematics, where the combination of "functional" and "flow" would be confusing. "Functional flow" refers to the order in which activities are carried out, with "flow" arrows indicating the reliance on the success of previous processes. As seen in the diagrams below, FFBDs can also express input and output data dependencies across functional blocks, but they are primarily concerned with sequencing.
