Tab Order for Data Fields on Overlays
Data fields on overlays are indexed separately from data fields on form pages. They appear under a separate Data Fields node beneath the Overlays node in the DataView tree. In the tree, you will see the page fields first, followed by the overlay fields in the order the overlays are arranged:

If you have more than one overlay, fields on the first overlay will be tabbed to first, followed by the fields on the second overlay, etc. Within each overlay, data fields will be sorted in the order you arranged them in the tree. Other types of fields also have their own nodes. These fields will have the same sorting hierarchy as they do on page fields.
Note:
In I-32 Featuring Editor 2, page and overlay fields can be included in the same tab order.