In Planbox every initiative has a Current iteration and a Next iteration. You can set the start date of the Next iteration on the Initiative Settings page. Optionally, you can create any number of future iterations. In addition, there is the Backlog, a special container in which you place items not yet planned. You can freely move items between Current, Next, Next+n and Backlog using drag and drop, or quick-menu actions.
When an iteration ends, Planbox performs a "Move to History" on the current iteration. It then becomes the last iteration. All the items that haven't been completed will be duplicated in the new current iteration and the duplicated item will contain all the remaining tasks. The item will also have a little number next to it that indicates the amount of iterations during which it has been ongoing.
The move to history happens automatically at the end of the iteration at 23:00 the last day of the iteration. Or you can do the move manually yourself on the last day of the iteration (on the Items & Tasks page, click on the Burndown tile and a link will allow you to do so).
Past iterations are always accessible on the Items & Tasks page.
In addition to the Next iteration, you can create future iterations by clicking on the down arrow in the Next tab. That shows you any future iterations. From there you can create a future iteration. Doing so will add a new iteration who's start date will follow the last. In it you can drag and drop items to plan that iteration.
From the drop-menu you can also delete the last future iteration. Doing so will move its items to the Backlog.