The Current IPTC Digest information is only present in image files that contain so-called IPTC information. It can be found mostly in image files such as camera raw images, JPG images or vector graphics.
The values found for the Current IPTC Digest are typically hashes that can be compared with other ICPT digests in order to check whether a file has been altered. These hashes are created using the MD5 algorithm that creates a 128-bit hash value which is primarily used as a checksum nowadays. The creation of this hash is done by the exif tool.
To understand the Current IPTC Digest, one has to also understand what IPTC is. IPTC is a widely used photo metadata standard. It defines the properties, fields, and structure of the metadata to facilitate the handling and access of images.