I'm not too sure of the correct terminology - but I hope this makes sense
panel code - the code that is triggered by the the HTML5 interactions (the CEP), debugged via remote chrome
application code - the code that can call the premier pro API (aka extendScript), debugged via the ExtendScript toolkit CC
First, let me summarize what (I think) I understand :
1) The panel code can call the application code using
Panels are basically web pages, hosted by Adobe's CEP (Common Extensibility Platform), which is an instance of Chromium.
Additionally, CEP provides an API that lets panels execute ExtendScript, in the host application's ExtendScript engine.