What happens when you merge 2 files together?
I assume your talking about merging 2 contacts together? Basically, the data on the contact record with the highest ID number takes precedence. The idea is that the highest ID contact record holds the most recent data on the record.
If there are empty fields on one record (e.g., there is no phone #) and that same field is present on the other record, the field containing data will take precedent.