Supply and Delivery

A Supply and a Delivery are boundaries, not tanks. They mark the two places where your question stops: where material comes from, and where it goes.

What they are

A Supply is where material enters the model. It has no level, no capacity and no heel, and it can never run dry. A Delivery is its mirror: material leaves there, and it can never back up or fill. Each one needs only a name.

Figure 1. A Supply feeds the model and a Delivery receives from it. Neither has a level.

Why a boundary beats a made-up tank

Without a boundary, a small model needs a made-up source vessel large enough not to starve, and a made-up destination large enough not to fill. Get either size wrong, and the model still runs, the numbers still look plausible, and the answer is wrong for a reason that is nowhere on the screen. A boundary cannot be sized wrongly, so use one wherever the real question stops.

Using them

Connect a pipe from a Supply into your first vessel to feed the plant, and a pipe from your last vessel into a Delivery to send material out. Both count toward the mass balance, so what came in through the Supply and what left through the Delivery sit on the same honest line in The Bulk report.