For the most part, all of the CDs you're likely to encounter are the same and can be read by the B2. There are discs, not officially CDs, that the B2 cannot read. It cannot read DVDs, Blu-ray discs, true MP3 discs, Super Audio discs, or DVD-A discs (which are technically DVDs, anyway). It's unlikely that most people would try to get the B2 to read those discs. But for those who would, there you have it.
There have been questions about MP3 discs. A true MP3 disc is a data disc (not an audio disc) which contains data files that are MP3s. The B2 cannot read a data disc. However, an audio CD-R can be made using MP3 source files. But in the case of the B2, there's no point in doing that, as the original MP3s can be copied directly to the B2 without need of burning a CD-R.
It has been requested that the B2 be made to read DVDs. It's quite unlikely that that will ever happen.