@romie75 Hello! There are probably many reasons why we convert region and product names to numbers. One I can think of is space efficiency in database systems such as SQL where instead of duplicating names for every record which can be very long, a number is used in place to reference that particular region or product, so instead of "americas" we would have "1". That is probably why in this challenge we are blending data with reference tables, because this is how some database systems work, with tables referencing other tables. In an SQL database we could drop the columns with the long names and keep the region/product key columns and have them be referencing their respective reference tables.
The "toString" function is there to convert the region/product key datatype to string so it can be written in the string "FileName" output.
@rijuthavposted a useful link to understanding the formula and output tools, especially for Excel, check it out here: