You can import structured PDF documents as artwork, which contains tags and structural information to provide a defined tabbing and reading order for assistive technologies such as keyboard access and screen readers.
The PDF Structure palette in LiveCycle Designer ES provides a view of the hierarchical structure of a tagged PDF and displays three types of structural tags:
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Content tags apply to the content within the document. Content tags are similar to HTML tags and are displayed as <P>, <H1>, <Sect>, <Figure>, and so on. The content of the document is displayed below these tags. Content tags are provided for reference only and cannot be modified in LiveCycle Designer ES.
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Field tags correspond to the actual objects in the document. Field tags can only exist under form tags. Each field tag can be linked to a single object. When a field tag is linked to an object, the tag displays the name of the object along with an icon indicating the object type in the document. When a field tag is unlinked from any object, the tag displays the words unlinked field.
When you import a structured PDF document as artwork, LiveCycle Designer ES maintains the structure and automatically associates any tagged Adobe Acrobat form fields that are converted to library objects with the correct form tags in the document structure. However, you may need to manually add field tags under the correct form tags for any untagged fields that remain in the form. You can also place additional library objects on top of PDF artwork and add field tags to the document structure for each object.
Using the PDF Structure palette, you can see which objects are associated with the structure tags in the form. When you click a content tag in the PDF Structure palette, the corresponding object in the form is highlighted in the Design View tab. For example, if you click a paragraph content tag <P> displayed in the PDF Structure palette, the corresponding content, usually text, is highlighted in the form.
After you import a structured PDF document as artwork, you can use the commands on the PDF Structure palette menu (and context menu) to add field tags to and remove them from the document structure, navigate the structure, move fields up and down in the structure, and show the content tags within the structure.
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Keep in mind that you can only edit the structure for fields in LiveCycle Designer ES. If you want to edit the structure tags and content structure in a form, you must do so in Acrobat.
See also 

About importing structured PDF documents as artwork