The 15 and 6 in the formula are nothing to do with COLUMN but are arguments of the AGGREGATE function.
I think 15 = Small and 6 = Ignore Errors. Don't quote me on that though.

Give it a try, in a blank cell type =AGGREGATE( then the intelisense will kick in and you can find those numbers in the list and it will show you what they're for.

BSB