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,

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

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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.

Thanks, I'd sought of figured it was something along the lines that _[Units] or whatever is part of the original and index wasn't.

What I'd tried first was to to it two stages, it was included in attachment,

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What first a group is added for the index and then another group for the sum as another column, in this case the index was straightforward but I could not

the units as a list I'd tried with each and _ , nothing, declare a variable (A)=> so A[Units] and referred to the name of previous table group column All;

[ALL] [Unit] ,

I've got as far as using (A)=> { A [Units] } , which returns 'list' , but each row is a list of one value so List .Sum just returns one value.

Thanks again for the help,
Richard.