A 
Part Page Component consists of a number of 
Fields, 
Tables, 
Controls, 
Resources and 
Scripts that can be reused in any number of 
Pages.
Part Page Components are often used for designing common page elements across an application. For example, headers, footers and menus.
You can add a Part Page Component either by right-clicking in the 
Outline or WYSWIG View and selecting Insert Control after.../Add Control to... > Part-Page Component:
You can test a Part Page Component by clicking the 
Test button in the Component editor's toolbar.