Layouts contain one or more visual elements, where those visual elements can be templates and certain types of portals. Layouts also represent a rectangular region within the containing panel or sheet. The layout is responsible for coordinating the position of its contained visual elements within its boundaries.
Layouts make it easy to group related visual elements together and position them as a atomic unit. For example, within the d20 System data files, there is a layout that manages the selection of class-specific special abilities, another layout for viewing the special abilities for the class, etc. By using layouts, new edit panels for custom classes can be quickly constructed by combining the appropriate layouts for the separate features of a given class.
IMPORTANT! Since layouts are not associated with a thing or pick, layouts may only contain portals that do not have field associations. This means that, in general, only label portals, image portals, chooser portals, and table portals may be used within layouts. In the case of labels and images, the field-based versions of those portal types may not be used within layouts, since no field association exists.