The EXIF Image Unique ID field is meant to store a globally unique identifier for a specific image file. In EXIF 2.2 it is defined as a 128-bit value written as ASCII hexadecimal, which is essentially the size and format commonly used for a GUID/UUID.

Its purpose is simple: give each image an ID that is extremely unlikely to match any other image. If generated properly, collisions should be extraordinarily rare.

In practice, though, usefulness depends on whether the camera or software actually writes the tag and how it generates it. The EXIF definition intends it to identify the image itself, not directly to store things like shutter count. Some systems may use their own method, but the standard idea is still the same: a unique per-image identifier.

So yes, it is intended to be unique across systems in a practical sense, but only to the extent that the device or software follows the spec and generates the value correctly.