Worksheets like the Inheritance Graph or the Application Graph can be used to edit a part of the project. E.g., there could be two Inheritance Graph worksheets, each containing only a subset of the whole project. For example, you could model Pump meta classes on one worksheet and model Valve meta classes on another worksheet. Of course, you can also have a worksheet showing all the models at once, if you prefer it that way.
So, a project can have multiple worksheets with overlapping content (i.e., models that exist in multiple worksheets). Changing a worksheet can affect other worksheets after saving the change in order to keep consistency(this happens automatically). You can save a (selected) worksheet to the project using the confirm button (the icon is a hook/check symbol) in the tool bar.
After saving the worksheet, the project will be marked as changed and can be saved using {{key press|Ctrl}} + {{key press|S}} or the save symbol in the tool bar, when the project page is selected.