TIFF Tag Reference, Private IFD Tags
If one needs more than 10 tags or so, the TIFF specification suggests that, rather than using a large amount of private tags, one should instead allocate a single private tag, define it as datatype IFD, and use it to point to a so-called 'private IFD'. In that private IFD, one can next use whatever tags one wants. These private IFD tags do not need to be properly registered with Adobe, they live in a namespace of their own, private to the particular type of IFD.
Exif tags are used largely to encode additional information related to image generation by digital still cameras
GPS tags are used to encode positioning information, related largely to image generation by digital still cameras
The single Interoperability tag is used to encode compatibility information, related to image generation by digital still cameras.