The second type of backup that Recovery Manager performs is an image copy^[1]. An image copy is similar to an operating system copy of a single data file. The difference is that an Oracle server process performs the copy, allowing for validation of the data blocks and registration within the control file. An image copy is not multiplexed, nor is any additional control information stored in the copy.
This is a physical copy so you can choose either a datafile, archived log file or control file.
All blocks are copied whether they contain data or not.

If you produce an image copy of a data file, you do not need to use Recovery Manager to restore this file. The switch command will update the control file, point to the backed up copy, and update the recovery catalog that the backup file is not available.
Recovery Manager will also support the reuse of image copies created by your operating system utilities. If you perform a copy of the USER tablespace data file to an alternate location, the recovery catalog can be updated to reference this copy, which can then be used in a restore or switch operation.

One of the benefits of using Recovery Manager to create an image copy (or backup set) is the ability to detect many types of corrupt blocks. Corrupt block information is stored in your control file and in an alert log file. You will be notified whether there is a logical or media corruption.
You can use two V$ views to view this information: V$BACKUP_CORRUPTION and V$COPY_CORRUPTION.

Click the link below to review backup sets.
Oracle Image Copies

[1]Image copy: An image copy is a backup of your database. An image copy can only be written to disk.