"Dolby Surround" refers to the 4:2:4 matrix stereo format developed by Dolby Laboratories, and brought to public attention when it was used on the 35mm optical-stereo prints of Star Wars. Based on the "quad" audio systems of the 1970s (the Tate SQ encoder was originally used), this takes four channels of audio, Left, Right, Center, Surround, & mixes them down into two channels, sometimes denoted L' & R'. Then, at the playback end, a special circuit takes the two channels & separates out four again. Of course this is only an approximate process ; you can't get four discrete channels out of two-channel audio!
The circuit used for "decoding" is termed "Dolby Pro-Logic" in consumer equipment. Of course you can use many other similar decoders for Dolby Surround, & many other similar formats (eg Chace Ultra-Stereo, DTS Stereo) can be handled by a Pro-Logic decoder. In fact, a Pro-Logic decoder will happily use the channel separation of an ordinary stereo recording to create four channels.
So, in this case the "format" is the standard analog or digital stereo track of LD, & the soundtrack is the theatrical "matrix" mix.