The Flow/Context layout is special because it has a specific behavior.
If you set the diagram layout to Flow/Context, the nodes that are in the Flow layout region, also known as flow nodes, are depicted as a left-to-right hierarchy in the lower half of the diagram. The context nodes are all in the top half of the diagram.
Note Flagging your nodes as flow or context nodes is always relevant, even when the diagram layout is not Flow/Context. For more information about how this is relevant, see When editing a diagram view, I can assign each node to either the 'Flow' or 'Context' layout region. What does that do? .