Page Layouts provide configuration for the visual layout of various components within an app.
The components that can be configured are:
Administration is provided in Admin: Page Layouts
Typically, each definition is assigned to an App Type. The current app type a user is using selects the appropriate definitions to apply.
A special case allows definitions to have a null app type, in which case the definition is available to users irrespective of their current app type (i.e. in all app types). Typically definitions assigned to an app type appear ahead of those with a null app type. For definitions where only a single definition can be used, one with an app type will override one available to all app types.