Beside the "main-channel" which holds all of the user-data, a CD-ROM contains a set of eight "sub-channels" where certain meta-information can be stored. (For an audio CD, the user-data is the audio itself; for a data CD, it is the filesystem and file data.) One of the sub-channels — the "Q-channel" — states the drive's current position relative to the beginning of the CD and the current track. This was designed for Audio-CDs (which for a few years were the only CDs), where this information is used to keep the drive on track; nevertheless the Q-channel is filled even on Data-CDs. Another sub-channel, the "P-channel" (which is the first of the subchannels) carries even more primitive information—a sort of semaphore—indicating the points where each track starts.