When the capacity plan is displayed by month or week, this button provides a view by day.
It is possible that the display granularity has been set by default via the checkbox. 
Default granularity of the 
Global configuration
This button allows you to return to a view when scheduling remaining work scheduling remaining work displayed by month or week. The last granularity displayed will always be the one displayed on your next visit. It is not possible to display days over a period of more than two months.
With the view , it is possible to quickly display the current month or the next three weeks by hovering over this line.
It is possible that the display granularity has been set by default via the checkbox. Default granularity of the Global configuration