what is the purpose of a CLN tape ?Removes muck from the tape drive's heads.
what if one keeps running the backup jobs for expired CLN tape ?You risk your backup jobs not copying data correctly to the tapes and/or not being able to read correctly from the tapes.
A cleaning tape can only be used a certain number of times, eg 15. Once the tape has been used 15 times, then it should be replaced, otherwise you risk putting more muck onto the tape drive's heads than you are taking off them.
I'm assuming you are using an auto-tape loader in which case, goto to the Devices tab and expand the list until you see your tape drive. Right click on the drive and from the menu that appears choose Clean.
Hopefully that's answered your questions, if not let me know.