What is Interoperability Index R98?

Question

In the uploaded pictures from my mobile phone, I get a line with the following information: Interoperability Index = R98? What is this Index?

Answer 1

Quote from www.exif.org/dcf.PDF (Design rule for Camera File system):

With the rapidly growing popularity of digital still cameras, there are increasing demands for file interchangeability, which will allow images captured on one camera to be viewed on another, or output directly to a printer. ExifR98 is a set of compatibility recommendations designed to meet such needs, by strictly defining the range of Exif application, and by setting out detailed rules for recording and playing image files not specified in the Exif format.

Basically it's a set of rules for the file names, directory structure and mandatory meta-data for digital camera so that all compatible viewers and printer will be able to read the memory cards of all compatible cameras.

All cameras that are compatible with ExifR98 are required to add the interoperability index tag to the EXIF data with the value "R98" (among other things).