Canvas Types in Oracle Forms 11g

 Oracle Forms provides four types of canvases, all of which can be displayed in the same window at runtime. A canvas' type defines how Oracle Forms will display it in the window to which it is assigned. When you create a canvas, you specify its type by setting the Canvas Type property. The four canvas types are: 

• Content 
• Stacked 
• Tab 
• Toolbar

Content Canvas :

The most common canvas type is the content canvas (the default type). A content canvas is the "base" view that occupies the entire content pane of the window in which it is displayed. You must define at least one content canvas for each window you create.

Stacked Canvas:

A stacked canvas is displayed atop-or stacked on-the content canvas assigned to the current window. Stacked canvases obscure some part of the underlying content canvas, and often are shown and hidden programmatically. You can display more than one stacked canvas in a window at the same time.

Tab Canvas:

A tab canvas-made up of one or more tab pages-allows you to group and display a large amount of related information on a single dynamic Oracle Forms canvas object. Like stacked canvases, tab canvases are displayed on top of a content canvas, partly obscuring it. Tab pages (that collectively comprise the tab canvas) each display a subset of the information displayed on the entire tab canvas.

Toolbar Canvas:

A toolbar canvas often is used to create toolbars for individual windows. You can create two types of toolbar canvases: horizontal or vertical. Horizontal toolbar canvases are displayed at the top of a window, just under its menu bar, while vertical toolbars are displayed along the far left edge of a window.

More details can be found at:
https://www.oracle.com/technetwork/developer-tools/forms/documentation/index.html