I was digging in the wonderful "menu" of goodies when you delve into configuring the CoolBar, and came across a whole group called "Source Notebook Commands"... which completely baffled me.
No... it's not about "Notebook" components (TNotebook)
Call me stupid... but as I wasted 15 minutes digging, I hope I can save the next person.
In this context, the "Source Notebook" is the basic "Source Editor" window of the IDE. When you move on to using multiple units, or if you trigger certain errors, multiple pages can open in the Source Editor window. THAT's all the "Source Notebook" is... the notebook of 1 or more pages, concurrently open in the Source Editor, between which you can "flip" by clicking the page's tab.
While on the subject... want TWO pages of sourcecode visible at the same time? I.e, say, your unit1.pas code AND your unit2.pas code? (Assuming you currently have them open in the source editor....) Right-click on a tab. Click on "Move To New Window". (Once it's there, to move it back: Close that window, use "File Open"... the relevant .pas file.)
Hope that is useful to someone someday. More such at
http://sheepdogguides.com/lut/index.htm