When using the field contexts (a.k.a. the "Applies to" setting in the field definition dialog) to target fields or groups of fields to certain contexts, you can filter the field definitions being shown in the list.
Click the "Filter" button to show a list of the currently used contexts:
The Filter button only shows when the list of currently used contexts is not empty
Once a context is selected, only field definitions that apply to that selection are shown. Field definitions that don't apply to the selected context are shown as outlines, so you are able to drag and drop the shown fields to a new location.
Click the filter button to remove the current filter and show all fields again.