Selfboot formats are the format in which the Dreamcast identifies CD-ROMs (not to be confused with GD-ROMs) as a valid game disc.
Originally discovered by Randy Linden of Bleam!, the Audio\Data format consists of a four-second audio track in the first session and an eXtended Architecture (XA) Mode 2 Form 1 data track that also includes hardware initialization code (also known as an IP.BIN file) in the bootsector, in the second session. This format was made open to public by Marcus Comstedt after Datel's Action Replay was commerically released and was then discovered and documented by Marcus to be using this selfboot format.
The other selfboot format, Data\Data was discovered by Xeal (or one of his online buddies). This format also consists of a XA data track, but instead of in the second session as the Audio\Data format was, the data track was in the first session. However, the second session was still a XA data track. It does not, however, contain any file data. It has the IP.BIN in the bootsector and the first 18 sectors of the first session data track padded out to the minimum track size (300 sectors).