The recovery of a database by applying all online and archived redo generated since the restored backup is called complete recovery. Typically, you perform complete media recovery when media failure damages one or more datafiles or control files. You fully recover the damaged files using all redo generated since the restored backup was taken.
Incomplete recovery differs in several ways from complete recovery. The most important difference is that incomplete recovery requires you to open the database with the RESETLOGS option. Using this option gives the online redo logs a new timestamp and SCN, thereby eliminating the possibility of corrupting your datafiles by the application of obsolete archived redo logs.
Because you must open RESETLOGS after performing incomplete recovery, you have to recover all datafiles. You cannot recover some datafiles before the RESETLOGS and others after the RESETLOGS. In fact, Oracle prevents you from resetting the logs if a datafile is offline. The only exception is if the datafile is offline normal or read-only.