Using application areas allows you to control the visibility of entire features in Business Central. When you use application areas, every control that you want to be shown when the application area is enabled must be tagged with the application area using the ApplicationArea property. A control can be tagged with multiple application areas.
As soon as you enable any application area, any control that is not tagged with an application area will always be hidden, so be sure that all your controls are tagged before enabling any application areas.