The SAP Docking Container allows you to attach a control to any of the four edges of a screen as a resizable screen area. You can also detach it so that it becomes an independent modal dialog box. Class: CL_GUI_DOCKING_CONTAINER.
If I have a requirement to display ALV output along with Selection screen on the same screen we can choose docking container.
SAP Custom Container: display controls in an area defined on a normal screen using the Screen Painter. Class: CL_GUI_CUSTOM_CONTAINER