what is this auto-complete / auto-reconcile feature in the editor and how do I know when it's needed and how to use it?

New to Appian, trying out the Practice projects, and have run into this feature more than a couple of times.  Since I don't understand what it is, I'm having trouble translating the exercises into keystrokes.  In the latest example of "typing blind:"

Where am I?

Online Course | Learning at Appian

Tutorial: Mobile Pocket Library

4 Interface Design

ShowBookDetails Interface ...

8. In the Component Configuration window under your rule inputs drill down into the columns by clicking Section Layout. Next, click the List of Components link under Contents. Click Columns Layout. Find and select the “Show dividers” feature.

9. Our first column will display the book’s cover image if it is available. To do this, drag and drop a WEB CONTENT field from the components tab into the top column.

○ In the Component Configuration window, change the label to Cover.

○ Edit the Source as an expression by clicking the pencil icon that appears on hover. You are going to make a reusable expression for retrieving the cover of a book with its ISBN-13 value.

Type or copy & paste the following expression: "">covers.openlibrary.org/.../" & ri!isbn13 & "-L.jpg"

What I tried to do:

copy & paste the following expression: "">covers.openlibrary.org/.../" & ri!isbn13 & "-L.jpg"

The error I got:

Note the ri!isbn13 variable (?) in the url expression

What eventually fixed it (sequence of 4 screen snips):

                    

   <<success!!!

So what is that step of auto-complete?  And when do I have to use it???  Copy-and-paste for the ri!isbn13 value didn't work.  But typing ri and then selecting the autofill suggestion, followed by selecting the subsequent autofill suggestion for book, followed by selecting from the autofill menu of fields, and eventually getting to the isbn13 field, resolved the references to allow the URL to be fully populated and the content retrieved.  

My questions are: when do I have to use this auto-complete / selection pattern, and how do I know when I need to use it?  Is there any separate documentation on using the Editor, or on the Appian UI, that tells me where I am, when I should be aware of how references (variables??) are being resolved,  and  how to go about using auto-complete to ensure proper resolutions???   Otherwise I'm trying to follow the practice tutorials and am simply not understanding how to make use of the environment.

Thanks in advance!  (pretty impressed with the tutorials and exercises so far, and Appian in general!)

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