The Key indicates which fields that import will search to find duplicate accounts. The import module will compare the information in the CSV file to existing accounts in the database. If the fields marked as Keys match existing information in the database, the import will return a matching account on Step 4. We can have up to three keys for each Name Format within a single import.
If trying to map more than 3 fields, users may get the error: "You can select a maximum of 3 fields when searching for matching accounts to use in your database. Using more fields will slow down the matching process, and create criteria which is too strict, leading to fewer, if any matches"