Secondary Issue Panels are the auxiliary grids, which can open next to the structure main grid. Even though they serve slightly different purposes, they work very similarly. They display a set of issues, which you can add to the structure grid using the drag-and-drop or cut & paste.
Structure has the following secondary panels:
You can configure the columns displayed on the secondary panel in the same way it's done on the primary structure panel - see Customizing Columns. The secondary panel configuration is the same for all secondary panels and is stored like the main panel's configuration.
You can divide the horizontal space between a secondary panel and the main panel by dragging the separating border.
Although they seem to work in the same way the structure panel works, you cannot edit or create issues on the secondary panel, nor can you move issues around there.