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What is the Image Unique ID in Exif info? what is it used for?!

It seems each system could use its own method, so is it still unique between different systems?

Searching online I was able to find this information:

Stolen Camera Finder
Apparently stolen camera finders use this Unique ID and other information to find stolen cameras.

Shutter Counts
Some forums suggests that Image Unique ID also contains shutter counts: Determining Shutter Actuations on an M8 and this one, so it's not always a random number.

Definition according to the Exif.org website:
"This tag indicates an identifier assigned uniquely to each image. It is recorded as an ASCII string equivalent to hexadecimal notation and 128-bit fixed length."

Also see this similar question on another forum.

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Based on the length of the field as defined in Exif 2.2, it appears to be a field to hold a GUID (or globally unique identifier) that should uniquely identify that particular picture from all other pictures. It should be chosen at random and based on the size of the available address space (128 bits) a collision is extremely unlikely for the foreseeable future.

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    \$\begingroup\$ AJ Henderson - you are thinking in the long term :-). If everyone on earth had a camera and took one photo a second, if a 128 bit binary number was used truly randomly then over a 100 year period there would be about a billion billion billion + unique numbers available PER SECOND per photo :-). ||| 2^128 / 8765 h/y / 3600 s/h / E10 people ~= 1E19. E11 people (!) = 1E18. \$\endgroup\$
    – Russell McMahon
    Commented Feb 13, 2013 at 22:10
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    \$\begingroup\$ @RusselMcMahon - true, but GUIDs are supposed to be unique across more than just photos. Camera's are hardly going to be the fastest consumer of GUIDs and eventually you will get collisions, but the chances of a practical, meaningful collision are effectively null for a very, very long time even so. \$\endgroup\$
    – AJ Henderson ♦
    Commented Feb 13, 2013 at 22:13
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    \$\begingroup\$ @RussellMcMahon - Your math isn't working too well for me. Based on current population, 2.24E17 GUIDs would be used per year based on all people shooting once per second (365 * 24 * 60 * 60 * 7097962000). That would require 1.52E21 years to consume all possible combinations (2^128/[previous result]). Our planet is going to be long relegated to space ash before we exhaust the set... having been consumed by our sun some 380 billion years prior. \$\endgroup\$
    – Joanne C
    Commented Feb 14, 2013 at 3:06
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    \$\begingroup\$ From the wikipedia link - "generating 1 billion UUIDs every second for the next 100 years, the probability of creating just one duplicate would be about 50%" That's not that far in to the future if we were going with the example of every person taking a photo every second. I'm not saying it's a real problem and this is really more of a question for Math or something similar. They are still practically useful, but they do eventually break down with enough use. \$\endgroup\$
    – AJ Henderson ♦
    Commented Feb 14, 2013 at 18:10
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    \$\begingroup\$ @Omne - it is possible in some cases it could be used for some other form of unique identifier (of which camera serial and shutter count would be unique). EXIF data is largely open to how the manufacturer wants to implement it, but the field is setup to be able to support GUIDs. \$\endgroup\$
    – AJ Henderson ♦
    Commented Jul 3, 2013 at 19:53

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