If most of the pages in your form design require a certain amount of standardized, fixed content, you can place that content on master pages to provide a consistent background and make editing easier. For example, you can arrange text, images, and geometric shapes on a master page to have them appear in the same location on multiple pages.
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If you are creating a form that contains subforms set to flow content, it is best to define the size and position of all content areas before you add additional objects to the master page.
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Objects can be placed anywhere on a master page, inside or outside the content area. As a general rule, do not place objects inside the content area unless you intend to have other objects placed on top of the master page objects when the form is rendered. If you are setting up a watermark, place it inside the content area.
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You will need text objects to hold read-only text, image or image field objects to position images (such as a logo), and you may consider adding geometric objects such as circles, lines, or rectangles to improve the appearance of the form design.
See also 

Headers, footers, and other background items