For example, in the sample Purchase Order form, you can see how the detailHeader and detail subforms are grouped into a subform set. The subform set keeps the two subforms together so that both subforms for each data item are rendered, or neither subform is rendered if no data exists for the detail subform.
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Use All Subforms in Order
All of the subforms in the subform set will appear in the final document and in the order they appear in the Hierarchy palette.
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Select One Subform From Alternatives
One subform from the set appears in the final document. For more information, see About choice subform sets.
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If you set a subform to a minimum count value less than the number of actual occurrences in the data, the remaining data values will not be displayed. Conversely, if you set a subform to a maximum count value greater than the number of actual occurrences in the data, the specified number of subforms are rendered, which means that some objects in the subform will render without data and appear blank.
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You can, if you want, nest subform sets within other subform sets to any number of levels. However, you cannot place individual objects such as lines, circles, or text field objects inside a subform set.
See also 

Overview of subform sets