Leave Upstream ID blank and a line follows the one above it; a plain series pipeline. Enter an upstream line's ID to branch off it. So: series by default, a tree when you need it.
Branched networks only, no loops
Every line has exactly one upstream line (a tree). This tool does not solve looped networks; those need iterative methods (EPANET or similar). Leaving loops out is what keeps it simple and exact.
No active pressure controls
You can add a fixed minor-loss valve (a k-value), but not pressure-reducing or pressure-sustaining valves (PRV/PSV). Their open/closed state depends on flow and pressure, which would force iteration.