Cropping groups and images
It is possible to crop (mask) a group of UI elements to only show a selected portion of it. When a group is selected, a Crop button will appear in the selection toolbar. Click on it to edit the visible area of the group.
Note: To crop a single UI element, select it and group it (yes, a group with only one element). You will then be able to crop it.
You can also use the crop tool to create image sprites, useful to create button state variants, for example.