The main part of the custom UI (as a reminder: it is a result type of the EPP) is quite simple:
it gets [1] the panels which are defined in [2] and specified in [3] and puts these together to the custom UI as horizontally split pane. There is no further action on that part.
Figure 513: custom UI source code (without other implemented methods)
The right panel [a] is unlike the left panel not an annotation interface, its construction is described on Section A18.7.4