Custom column Index or Ranking by other column

Thats like an index on a table partition. You can create that by using grouping on the column and returning "_" - which means that all column of the table (but only for the specific value in the column) will be return. You then nest your Index-command in:

let
    Source = Table1,
    Partition = Table.Group(Source, {"Group"}, {{"Partition", each Table.AddIndexColumn(_, "Index",1,1), type table}}),
    #"Expanded Partition" = Table.ExpandTableColumn(Partition, "Partition", {"Date", "Index"}, {"Date", "Index"})
in
    #"Expanded Partition"