You work with content areas on the master pages to control the areas and flow direction in which LiveCycle Designer ES places objects on the pages. Remember that you cannot work with content areas on the pages.
Every master page has a default content area, which is displayed on the associated pages and outlines the area in which you can place objects on the pages, much like the margins in a text document. You cannot place objects on the pages outside the area represented by the content area.
However, the content area does not delineate or limit the area in which you can position objects on the master pages. In fact, you can place boilerplate objects anywhere on the master pages, inside or outside the content area. If you do place objects outside the content area to contain header and footer text, the date and time, or page numbering, you need to test the form to ensure that the objects appear as intended. You want to be sure that objects on the pages do not overlap and hide the objects on the master pages when the form is rendered.
With forms that have sections that adjust to accommodate data, remember that the layout of the form is ultimately data-driven. If the form is being rendered through LiveCycle Forms ES or LiveCycle Output ES, the pages are added until all the available data is merged. As new pages are added, data flows from page to page within the area defined by the content area on the master pages.
If you want the same basic layout for each page in your form, you may find that the default content area is all you need. However, if the layout is more involved, you can add additional content areas to the default master page. Keep in mind that if you want different flow direction, page orientations, or two-sided pages, you can insert additional master pages and configure the other master pages and content areas to suit your design.
See also 

Using content areas in forms that have a flowable layout