I am really missing the purpose of what the digit does!?

Essentially, the unpack just returns a list of values that you have to assign in some meaningful way.

The digits in the template are just telling unpack how to parse the input data. No provisions are made to assign "sub-lists" to separate arrays, or some such.  In other words, in the example "C2 C4 C" has the same effect as saying "C7".  But note that in other cases, such as "A2 A4 A", where A2 etc. would return a single element, there would a difference to saying "A7".