Several bits of data, including the Disc Application Code, exist in the Wobble Track
of a CD-R
The purpose of the Disc Application Code is to reveal what the accepted use of a CD is. There are codes for general use (data CDs), unrestricted use (audio CDs), as well as a number of restricted use types (audio CDs, test CDs for Sega Saturn, Kodak Photo CDs, etc.)
Most older CD players will play any media inserted, however some newer models will refuse to play any CD that has a wobble track (which is to say, any CD-R) and that doesn't present an unrestricted use (audio) disc. Early on, the DVD Consortium required that all DVD players recognize and obey the disc application code, however as even Sony's newer DVD players will play general use (data) CD-Rs, presumably this has been rescinded or merely ignored owing to the inconvenience and lost unit sales.
When you purchase an unrestricted use (audio) CD, you're actually paying royalties to most labels represented by the RIAA on the assumption that you'll be using the disc to press their music. Unfortunately, there is no refund mechanism in place if you use the CDs to press music not under license of the RIAA's clients.