When more than one content area has been defined, objects flow from one content area to the next when the form is rendered. You can select a flow direction for the objects and their data. The flow direction controls when an object receives the focus relative to the other objects in the form during a tabbing sequence. It also determines when an object receives merged data relative to the other objects in the form. Specifying the flow direction of one content area automatically sets the flow direction of all other content areas in the form.
When the form is rendered, if an object that merges data expands so much that it will no longer fit in one content area, the object spills into the next and subsequent content areas. Similarly, when you are designing a form, you cannot add more objects to a content area than will fit within the area bounded by the content area. As soon as the content area has been filled, the remaining objects overflow into the next available content area. If another content area is not available to catch the overflow, the data will not be rendered properly.
See also 

Data flow between content areas