My understanding is:
- you can't select multiple rows for deletion. If you know what the characteristic(s) are that the rows have in common, you could do a 'Find' for that and delete the found set.
- you can do 'Omit Multiple', so if you sort by the criteria that would mark a record for deletion, you could omit in batches all the ones you don't want to delete and then delete the records remaining.
- there are some actions that you cannot undo. Among them is 'Replace Record Contents'. I believe that some aspects of record importing would also be undo-able. Although when you import new records the imported set if by default the new found set, so if it is wrong you can immediately delete it. Importing by 'Updating Current Records' is, I believe not undo-able. As is 'Update Found Set'.
- Taking a copy before importing would help alleviate this problem, of course.