During the import process, if you have new accounts that do not return a match on the Possible Duplicates Report step, you have the option to either Create a New Account or Skip Those Lines. By choosing Skip Those Lines, you are not taking any action on those accounts in your file that do not have an identified duplicate already in the database. You may want to have a list of those accounts with no match so that you can review them, and choosing Skip Those Lines gives you the option of producing that list.
On 'Step 4: Possible Duplicates Report' of your Import, select Skip Those Lines in the Options section where it says Lines With No Matching Account. Once you've made any other selections you need regarding your duplicates, click Next (or click Step 5: Run Import). On that screen you can then click Download an Import File of Skipped Lines to get a file of all of your skipped accounts.