Tabs - PdfTypeName - (Optional; PDF 1.5) A name specifying the tab order to be used for annotations on the page. The possible values are R (row order), C (column order), and S (structure order). See Section 8.4, “Annotations,” for details.

8.4, “Annotations”
77. In PDF 1.5, the order of moving the keyboard focus between annotations
on a page by using the tab key can be made explicit by means of the page’s
Tabs entry (see Table 3.27). In earlier versions, the tab order was not explicitly specified and depended on the viewer. In Acrobat 4.0, the order includes only widget annotations and is determined by their order in the
page’s Annots array. In Acrobat 5.0, the order includes all annotations:
widgets come first and are ordered as in Acrobat 4.0; other annotations
come after widgets and are ordered by rows. Acrobat 6.0 has the same behavior as Acrobat 5.0 for documents that do not contain a Tabs entry. For
documents that have a Tabs entry, Acrobat 6.0 reorders widgets in the
Annots array to match the specified order (row, column, or structure) so
that the tab order for widgets is preserved when the document is opened
by earlier viewers.