.iso vs .img files for DVD images?
Single-track (.iso) vs. multiple-track (.img)


  Any single-track CD-ROM, DVD or Blu-ray disc can be archived in ISO
  format as a true digital copy of the original.
  
  But ISO files have limitations. For example, a CD can have multiple
  tracks, which can contain computer data, audio, or video. File systems
  such as ISO 9660 are stored inside one of these tracks. Since ISO
  images are expected to contain a binary copy of the file system and
  its contents, there is no concept of a "track" inside an ISO image,
  since a track is a container for the contents of an ISO image. This
  means that CDs with multiple tracks can't be stored inside a single
  ISO image; at most, an ISO image will contain the data inside one of
  those multiple tracks, and only if it is stored inside a standard file
  system.
  
  Formats such as CUE/BIN, CCD/IMG and MDS/MDF formats can be used to
  store multi-track disc images, including CD-Audio discs. These formats
  store a raw disc image of the complete disc, including information
  from all tracks, along with a companion file describing the multiple
  tracks and the characteristics of each of those tracks. This would
  allow an optical media burning tool to have all the information
  required to correctly burn the image on a new disc.


Shamelessly copied (an slightly edited) from wiki/ISO_image (24-10-2015).