When you select a layout tab, you access an area called paper space. In paper space, you place or edit your title block, create layout viewports, and plot your drawings. Although you are in a layout tab, you can still modify entities in your model tab. It is important to look at the UCS to see which space you are working in while in a layout tab because it's icon will change. You can use the "ms" command and the "ps" command to switch between the two different spaces while in a layout tab. It is also important to note that you will only be allowed to switch between the two spaces if you have a viewport setup in Paper space.
Paper Space in a layout tab.
While in paper space, a triangle UCS is displayed in the corner of the screen.
Model Space in a layout tab.
The triangle UCS is gone, and you have the regular UCS in the model space view.