What is the difference between a backup datafile in a tablespace and a datafile in a tablespace which is made READ ONLY just after the backup is taken?
No updates are made in the tablespace after backing it up, but a command for making the tablespace READ ONLY is executed.
Do the commands for READ ONLY and READ WRITE cause any change if the datafile or the datafile is similar to the backed up file?
When you make the tablespace READ ONLY, the SCN in the datafile header is frozen. If your backup occurs before the READ ONLY command was issued, then on restore, Oracle will know that the SCN in the backup is earlier than when the file was made READ ONLY. While you know that no transaction has modified this file, Oracle does not have this information. So recovery would need to be applied. It is always a good idea to take a backup
after one makes a tablespace READ ONLY.