TRAY container is an UI element, we can insert other UI elements into it, using this we can format design of a web dynpro screen(view).
Step1: Go to SE80, create a web dynpro component ZSAPN_TRAY_CONTAINER.
Step2: Go to Main view, go to layout, right click on rootuielementcontainer ->insert element, provide id as TR1, type as 'Tray'.
Step3: Group is inserted, by default caption will be created for group, select CAPTION, provide text under properties, we can set borders for this.
Step4: Now you can insert other UI elements into it.