How can I create an autonumber field that only increases when the column "Hierarchy" = 0?

Peace

What I have tried so far but is not working:

1. =IF([Hierarchy] = 0, MAX((IF([@[Hierarchy]] = 0, [@[AutoNumber]]))) + 1, "")
2. =IF([Hierarchy]@row = 0, COUNTIFS([Hierarchy]:[Hierarchy], 0, ROW([Hierarchy])@row , "<=" & ROW()), "")
3. =IF([Hierarchy@row ] = 0, COUNTA([Hierarchy@row ]) + 1, "")

Peace

OP: Ideally I would want to add the autonumber to a prefix but I could add a concatenate formula to it afterward.

Paul Newcome

You would first insert an auto-number column (called "Auto" in this example and formatting doesn't matter). Then a text/number column called "Row" with this column formula:

=MATCH(Auto@row, Auto:Auto, 0)

Then in the column you want the incrementing number to be in, you would use

=IF(Hierarchy@row = 0, COUNTIFS(Hierarchy:Hierarchy, Row:Row, @cell <= Row@row))