AutoCAD switches from model space to paper space when you are working on a layout tab.
On the layout tab, use paper space to create a finished layout of a drawing for printing. As part of designing your layout, you create layout viewports, which are windows containing different views of the model. By switching from paper space to model space (see MSPACE), you can edit the model and views within the current layout viewport.
You can make a viewport current by double-clicking inside it. You can switch to paper space by double-clicking an area of the paper space layout that is not within a viewport.
You can also switch between model space and paper space by choosing Model or Paper on the status bar.