Residual Edge is an edge which flow to the opposite of the flow edge#. Similar to the flow edge, it has both flow value and maximum capacity in the format of flow value/maximum capacity. However, it is initialised to 0/0 unlike its counterpart.
The sole existence of Residual Edge is to undo or revert bad #202206091601 which doesn’t lead to #maximum flow. The Residual Edge is valid to be included into the augmentation in the later iteration (the edge has been taken at least once). When reverting, add the flow value of the residual edge to the flow edge. It is to avoid having a negative remaining capacity# (the difference of the flow edge’s maximum capacity and flow value).