If your form uses the default tabbing order, the tabbing order will be in geographic order, left-to-right, top-to-bottom. Tabbing order respects the existence of subforms, radio buttons, and content areas. Tabbing order is also determined by the vertical position of objects on a page and its master page.
Setting a meaningful tabbing order is very important when designing forms that are accessible to users with vision impairment or disabilities. These users typically do not use a mouse to navigate through a form, so they depend on the keyboard. A logical tabbing order sequence ensures that they have full access to all the fields on the form and that they can navigate the form in a way that is sensible and efficient.
The tabbing order also determines the reading order in which the screen reader reads form objects. The reading order is based on the PDF logical structure that is created when you save the form as tagged PDF.
LiveCycle Designer ES includes all static objects and field objects in the logical structure. Make sure all text and image objects are placed logically in the tabbing order so that the text within the text object and any tool tips or custom screen reader text set for the image are read at the appropriate time.
Because the tabbing order is important, ensure that you position objects precisely on the form, relative to each other. For example, you can position and size an object using its coordinates and you can snap objects to points on a grid.
If you create a large interactive PDF form with no structure, end users may experience slow performance in tabbing between fields. This problem is averted if you save the form without tagging. However, if you need your form to be accessible, you must save it as tagged PDF. To work around this situation, you can wrap sections of the form in unnamed subforms. This step adds the required structure to the form.
See also 

Tabbing order for accessible forms