Good question. Performed a quick search in community and documentation and did not get any relevant hits. Even searched for tabindex. Tab order seems to be top-to-bottom and left-to-right on a form and left-to-right within a table.
Is your Field Editor in Edit mode? Normally the tab order is DOM order, so it ends up being top-to-bottom within the editable Fields in a Section, and from one section to another, then left-to-right going between Columns.