INSERT only
The IGNORE_DUP_KEY option controls what happens if execution of a multi-row INSERT causes violation of uniqueness in an index. Default behaviour is to give an error message and undo the entire INSERT statement, but with IGNORE_DUP_KEY on, a warning is given and only the offending rows are refused and other rows are still inserted. The DELETE statement is of course not affected, since deleting rows can never cause violations of a unique index. The UPDATE statement could have been affected, but Microsoft has chosen not to do so. The MERGE statement is also unaffected by this option, though this is not documented very well, and I suspect this to be a bug.