Canvases are displayed in windows at runtime; the canvas to which an item is assigned determines the window in which the item will be displayed. When an item is created by drawing it in the Layout Editor, that item is automatically assigned to the Layout Editor's current canvas. To display an item at runtime, assign each interface item in a form to a canvas by setting the item's Canvas property.
When the Canvas property for an item is left unspecified, that item is said to be a NULL-canvas item. NULL-canvas items are not displayed at runtime or in the Layout Editor at design time. You can use NULL-canvas items programmatically as variables, by assigning and referencing their values in PL/SQL code using :block_name.item_name syntax.
To make an item a NULL-canvas item, simply leave its Canvas property blank.