Power Query Grouping Column as list

Hi,
I've managed to get the result I wanted but aren't sure how ;
I've attached workbook showing all examples;

I've grouped some data all rows and then within that wanted to do a running total using list sum with list firstN and index.

this is what I've got,

Table.Group(  Source, { "Item"} , {{"All", each

          Table.AddColumn ( 

                   Table.AddIndexColumn( _ , "Index", 1,1,Int64.Type),

                                   "new", (A)=>   List.Sum( List.FirstN(  [Units] ,  A [Index]     )) ) }} )

What I would like some help in understanding is if you just put in [Units]

 Table.AddIndexColumn( _ , "Index", 1,1,Int64.Type), "new", (A)=> [Units] ) }} )

you get a list but if you do the same with index you get an
error? it has to be prefixed with the function ; can someone explain or point me in the direction of information other than MS docs.
also I realise this is probably not the best way of approaching this.


Richard.