This tab is available when you select the first slide. It allows you to choose the transition that is visible at the start of the banner.
Because this transition is a bit special (in fact it does not include the beginning of the transition, where the previous slide is supposed to disappear), the result can sometimes be a bit surprising, depending of the transition used. So test for yourself if the banner displays the desired effect.
If you have a banner that "loop" (Back to the first slide when the banner comes to an end), make sure you chose the same transition in the last slide that the initial transition to have a coherent transition.