UPC stands for 'Universal Product Code'. It is an international physical bar code. And ISRC stands for 'International Standard Recording Code'. It is an international code which gets embeded as data into your recordings and works as virtual identification (think as an international bar code for the recording). Without an ISRC issued and attributed to your recording, it does not have any trail where to forward the royalties to, thus, it could be played for free without knowing it's to you the royalties should go to.