Most of the time you edit data in data view, since it shows only the data in your map and hides the layout elements. You can also edit in layout view, although editing is typically easier and more accurate in data view. However, editing in layout view is useful when you want to make minor additions to your map in the context of your map layout.
When editing in layout view, it is recommended that you work inside (focus) the data frame. To focus a data frame, double-click it with the Select Elements tool 
. You might experience unexpected results if you do not work in a focused data frame, especially if you have any layout graphics. The reason for this is that the ArcMap editing tools always work inside the data frame; however, the ArcMap graphic tools and keyboard shortcuts can work in both the layout and inside the data frame. For example, suppose you are editing in layout view but not working inside a focused data frame, and you have some features and some layout graphics selected. If you press the DELETE key, the selected layout graphics will be deleted. This unexpected behavior can be avoided by working inside the data frame when editing in layout view.
Often when working in layout view and at certain scales when a data frame reference scale is set, the outline of selected features and other editing and display feedback items appears too large and obscures the features. Therefore, these items ignore the reference scale and always appear at a constant size.