Microsoft Visio 2013 : Adding Structure to Your Diagrams - Sizing containers

As you have already learned, when you drag a shape into a container, a green outline appears on the border of the container. This is true even when you drag most but not all of the shape into the container. In the following graphic, the wireless access point will be added to the container when the mouse button is released, even though it is not fully within the borders of the container. (Depending on the resize options described just before this sidebar, the container may expand to encompass the new shape.)

The following graphic on the left shows a different container behavior that you might use from time to time. In contrast to the previous graphic, the wireless access point has not been dragged quite as far into the container. The container signals the difference by displaying a green outline only on the top border and not all the way around. When the shape is dropped, the wireless access point will become a member of the container, but it will be attached to the edge of the container.

The key difference in behavior of a boundary shape is that when the container is resized, the boundary shape stays on the border. For example, when the resize handle on top of the container in the right-hand image in preceding graphic is dragged upward, the wireless access point remains on the container boundary.

You might use border shapes for a situation like the one shown in this network diagram because the wireless access point is attached to the branch office network but is also linked to other networks. Consequently, it makes sense to show it on the border rather than in the interior of the container.