# Overview of Sub-models

This section provides a brief introduction to Sub-model capabilities. In it, you'll learn about what a Sub-model is, and how one operates. You'll also learn about how the diagram display can change when you open, and subsequently close, a Sub-model.

A Sub-model is a device for encapsulating detailed structure within a model. Without Sub-models, the stock and flow structure of a model will have a "flat" appearance on the diagram. As suggested by Figure 9-2, flatness can make a model difficult to "read."

Figure 9-2 Without Sub-models, the Diagram can be Visually Overwhelming

In Figure 9-2, it's hard to know where to start. Because the entire model exists on a single layer, there is no way to differentiate between the details and the essence. With a highly detailed model that spans several pages, the problem can become even more severe.

By encapsulating structure, Sub-models add layering to the diagram. In so doing, Sub-models make it much easier to separate the essence from the details. As an illustration, Figure 9-3 takes advantage of Sub-models to represent the same process as shown in Figure 9-2. This simpler representation captures the heart of material flow from the supplier, through the firm, and to the customer.

Figure 9-3 Sub-models Simplify Diagram Display

Note the large, conveyor-like icons for the Supplier and the Firm. These are called Sub-model icons. Within each icon, you'll find a Sub-model. Sub-models can be opened or closed selectively, providing you with access to the details of the underlying process.

For example, imagine that you wanted to drill down into the logic of the Supplier order fulfillment process. Double-clicking on the Supplier icon will open its Sub-model, as illustrated in Figure 9-4. After exploring the Sub-model, you can click on the up-arrow that connects the Sub-model space with its icon. The Sub-model will be closed once again.

Figure 9-4 Selectively Display a Sub-model

Layering introduces the potential for Sub-models to obscure main-level structure. For example, consider the map in Figure 9-5. The stock of Workers is used to drive the production activities within the Work in Process Sub-model.

Figure 9-5 A Typical Sub-model Configuration

Clearly, there is potential for a topological difficulty with this diagram. When the Sub-model icon Work in Process is opened, the Sub-model that will be ejected will lay right on top of Workers.

Or will it? In designing the Sub-model feature, we anticipated the potential for Sub-models to obscure main level structure. To get around the problem, the software allows all main level building blocks and objects to have two positions. Each building block and object will remember the two positions you have set for it. The first position is the position you establish when no Sub-model spaces are open. The second is the one you establish when one or more Sub-model spaces are open. When you open any Sub-model, all main level building blocks and objects will move to their second position. When you close all Sub-models, all main level building blocks and objects will locate themselves in their first position.

Continuing with the illustration, Figure 9-6 shows the result of opening Work in Process.

Figure 9-6 Two-position Display of Diagram Elements

Notice that the structure for Workers, hiring and layoffs, and target inventory was "pushed aside" when the Sub-model was opened. These building blocks were "pushed aside," because a second position had been previously set for them while the Sub-model was open. Now, the structural elements know both their first (all Sub-models closed) and second (any Sub-models open) positions. Closing the Sub-model will cause the structure to move back to its previous position. Opening the Sub-model will push the structure aside once again.