Often boards are added to the certificate that doesn't reflect how they form part of the installation. This feature allows you to re-irder the boards to how you want them.
The original board order
Looking at the DB selector will show you the current order of boards - if you don't want it like that, click the Move boards button.
Re-order the boards
Using the three handles icon click and hold each board and drag it to where you want it, then let go of the mouse. Click Update board order.
New board order
The DB selector will now show the new order - the boards will also print in this order.