An energy flow diagram shows the movement of energy used by a specific consumer or set of consumers from source to usage. In the example below, the flow energy is shown for the United States.
The diagram tells us that 37% of primary energy used in the U.S. comes from petroleum. Then, by looking at "percent of source type" we can say that 70% of the petroleum used in the U.S. is consumed by transportation. We can look at "percent of usage sector" to determine that 91% of transportation energy use if fueled by petroleum. And lastly, we can say that transportation uses consume 28% of total energy.