I just received an explanation from my Danish host provider. I include it here, in case anyone else wants to know:
When you do an export, a sql script is being generated, to be used when recovering your database. Here's how the 3 types work:
Insert: Inserts records from the exported file (regardless if they're in the database already or not).
Update: Updates those records that are in the database already. This means that if a field is empty it will be updates - if it's not empty then it won't be affected by the exported record.
Replace: Overwrites the records that are in the database, but does not insert records that have been deleted.
Insert is recommended in most cases (and is defaut too).