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A VideoCD or SuperVideoCD data file. Typically a .dat file found on VCD or SVCD disc contains MPEG-1 (in case of VCD) or MPEG-2 (SVCD) -encoded video and audio encoded in MP2 format.
However, as VCD and SVCD can contain variety of other content as well, it is easier to simply state that .dat contains whatever data is there on a disc for that particular track -- it can be a normal, plain audio/video data, but it can also contain subtitles or photos (JPEG -compressed).
VCD .dat files can be played with virtually all standard video software players, but in order to play SVCD .dat files, the system needs to have an MPEG-2 codec installed or the player has to have an integrated MPEG-2 decoding that supports SVCDs (one such player is open-source player called Media Player Classic).
Related software tools
Media Player Classic (Open source)
Media Player Classic is an extremely light-weight media player for Windows. It looks just like the good-old Media Player v6.4, but has lots of nice extra features.