Creating a new listing with flat files

I’m just starting to dabble with flat files and wanted to know if it’s possible to create entirely new parent listings with multiple child variations in the same file. What am I supposed to put for SKU and ASIN if a SKU and ASIN hasn’t been created yet?

Or are flat files used to organize listings I’ve already uploaded manually into Amazon under existing parents?

I have 115 items with 26 size/color variations each so I’m trying to upload them all the fastest and easiest way possible. Thank you.