For example, assume we add a button to a parent object named button1 and then another button named button2. Then button1 (along with its child object(s)) will be in the background and can be covered by button2 and its children.
- Use `lv_obj_move_up(obj)` to move an object one position up in the hierarchy, Similarly, use `lv_obj_move_down(obj)` to move an object one position down in the hierarchy.
Both are visible and common on all screens of a display. **They are not, however, shared among multiple physical displays.** The `layer_top` is always on top of the default screen (`lv_scr_act()`), and `layer_sys` is on top of `layer_top`.
The `layer_top` can be used by the user to create some content visible everywhere. For example, a menu bar, a pop-up, etc. If the `click` attribute is enabled, then `layer_top` will absorb all user clicks and acts as a modal.