Explanation
Tab order is the order in which users interact with the items on a page using the keyboard. Generally, we want tab order to be predictable and to closely match the visual order on the page (unless there is a good reason to deviate).
Note: If you are using the keyboard to navigate in a Power BI report, the order in which you arrive at visuals will not follow your vision unless you set the new tab order property. If you have low or no vision, this becomes an even bigger issue because you may not be able to see that you are navigating visuals out of visual order because the screen reader just reads whatever comes next.
Reference: https://datasavvy.me/2018/12/26/tab-order-enhances-power-bi-report-accessibility/