Imke Feldmann shows off what you can do with the fifth parameter in Table.Group:
The Table.Group-function will pass 2 parameters to the function in the 5th arguments if it is used: For GroupKind.Local this is group-columns-record from the initial/first row of the table/group and the respective record of the current row.
As long as the comparer-function returns 0, the current row will be regarded as belonging to the group: This is a match in the Comparer.OrdinalIgnoreCase-function and also the value of false (which makes the syntax a bit counterintuitive here in my eyes)
Interesting reading.