"audio CD" blanks are basically regular CDRs with a special tax added to the price, supposedly to pay back the music industry for the fact that people use them to copy audio, rather than buying another copy. 🙂 However they also contain a bit of information in the ATIP (Absolute Time In Pre-groove) -- which also stores data for a burner telling it what type of dye is used, how much data the disc can hold, et cetera -- which marks them as audio CDRs. A standalone CDR burner will only burn to that type of disc (so that it guarantees that anybody that's using a standalone burner, which is only used to copy CDs, is paying the tax on audio discs).