The XCenter "Tray" widget allows you to group other widgets into so-called trays and quickly switch between several trays. Each such tray within a tray widget can be considered a "mini-XCenter" inside the tray widget. It has an "Add widget" menu, and you can drag objects on it.

These trays are very similar to those of the WarpCenter. As opposed to the WarpCenter though, there are no limits as to how many trays can be defined per tray widget, and you may add more than one tray widget to each XCenter.