The element is edited the normal way (default).
If acting on the element as a whole (NONE), this will MOVE the element by MOVING its toplevel. When acting on the start of the element (START), this will only MOVE the element, but not its toplevel parent. This can allow you to move a Clip or Group to a new start time or layer within its container group, without effecting other members of the group. When acting on the end of the element (END), this will END-TRIM the element, leaving its toplevel unchanged.