A point in the task graph: one task, one task group, or a set of them.
Flow declares a dependency where nothing flows but the ordering. An edge that carries a value is declared by passing the upstream's handle instead.
Runs the tasks here after each of previous, carrying no value.
Runs the tasks here before each of next, carrying no value.
The ends an edge drawn here attaches to. A task stands for itself, so the default is nodes; a task group stands for the group rather than for the tasks it holds today.
The tasks at this point in the flow.