The context diagram and the activity diagram are not necessary, but help you visualize the use case and its position in the project.
环境图和活动图不是必须的，但是可以帮助你可视化用例和它在项目中的位置。
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As Figure 15 shows, the page boundaries are indicated in the outline view and on the process diagram, enabling you to position the elements of your process so that they don't fall on page boundaries.
Each usually comes with a diagram, marking the position of the tiny specks of carbon that can be seen when viewed through a "loupe", the jeweller's 10x magnification tool.